Called in TDSet::SetEntryList fixes ROOT-8165 .

Do not try to process websocket close synchronousely. 
It is anyway last action with websocket and does not require that
calling thread blocks. Additional protection for the case of shutdown 
of THttpServer with running websockets

THttpCallArg object, submitted with ownership, will be destroyed by the
THttpServer

Seems to be, has effect on older GCC versions

See https://github.com/civetweb/civetweb/releases for more details

Without this dependency, a change to one of LinkDef{1,2,3}.h will
not trigger the regeneration of the G__Core.cxx dictionary, since
only the main LinkDef.h header was explicily listed.

Original commit message:
"Constify. NFC."

This commit backports the following LLVM patch:

https://github.com/llvm-mirror/llvm/commit/061e7017039f963caff79bff51c0fb43a88bebdd

which also fixes compilation of ROOT with the Intel Compiler.

Formally is not required. Could help to detect cardinal failure early

Shift termination to the ProcessRequests.
Keep agreement, that all central calls performed from main thread,
which suppose to be when THttpServer::ProcessRequests() is called

Otherwise it could happen, that calling object (websocket handler) will
be destroyed by such action. 

Like now civetweb is blocked, waiting that websoocket request is
processed, but gApplication->Terminate() starts deleting of all
connections and trying to call mg_stop.

It should terminate THttpServer first

When exit ROOT application, one should call THttpServer::SetTerminate()
to prevent any further http processing. Otherwise TCivetweb may hang in
exit sequence, while THttpServer cannot process some closing requests
properly

At this moment ROOT event loop may not be active and therefore requests
are not processed. Problem appeared with the opened websockets -
civetweb invoke close handler at the end, which hangs

See 7fca26b9.

We need to first Close the file before deleting the interpreter object,
otherwise the script:

   auto f = TFile::Open(filename,"RECREATE");
   TH1F h(...);
   h.Fill();
   // etc...
   .q

would result in an 'empty' file.

However, when closing the file, we can not delete the object its own and some interpreter
static object (for example a MakeClass skeleton) might hold pointer to those objects.

To resolve this, we introduce a new flag to TDirectory/TDirectoryFile/TFile Close ("nodelete")
that will write the data in to the physical file but will not remove the object from memory.

valgrind --suppressions=/opt/build/root_builds/rootcling.cmake/etc/valgrind-root.supp root.exe -b -l -q execLHEF.C
==6337== Memcheck, a memory error detector
==6337== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==6337== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==6337== Command: root.exe -b -l -q execLHEF.C
==6337==

Processing execLHEF.C...
direct
Warning in <TClass::Init>: no dictionary for class TRootLHEFEvent is available
Warning in <TClass::Init>: no dictionary for class TRootWeight is available
Warning in <TClass::Init>: no dictionary for class TRootLHEFParticle is available
Warning in <TClass::Init>: no dictionary for class TSortableObject is available
all=79618 zero=60000 low=9599 high=10019
reader
all=79618 zero=60000 low=9599 high=10019
legacySelector
Info in <TTreePlayer::MakeClass>: Files: lhef_leg_sel_gen.h and lhef_leg_sel_gen.C generated from TTree: LHEF
all=79618 zero=60000 low=9599 high=10019
selector
all=79618 zero=60000 low=9599 high=10019
makeClass
Info in <TTreePlayer::MakeClass>: Files: lhef_mc_gen.h and lhef_mc_gen.C generated from TTree: LHEF
all=79618 zero=60000 low=9599 high=10019
(int) 0
==6337== Invalid read of size 8
==6337==    at 0x1636165F: TTree::GetCurrentFile() const (TTree.cxx:5247)
==6337==    by 0x405F343: ???
==6337==    by 0x405E6DB: ???
==6337==    by 0x405E6AB: ???
==6337==    by 0x911919A: cling::IncrementalExecutor::runAndRemoveStaticDestructors(cling::Transaction*) (in /mnt/vdb/lsf/workspace/root-pullrequests-build/build/lib/libCling.so)
==6337==    by 0x90C7A3D: cling::Interpreter::runAndRemoveStaticDestructors() (in /mnt/vdb/lsf/workspace/root-pullrequests-build/build/lib/libCling.so)
==6337==    by 0x8FB13E2: TCling::ResetGlobals() (TCling.cxx:3237)
==6337==    by 0x52C4915: TROOT::EndOfProcessCleanups() (TROOT.cxx:1193)
==6337==    by 0x54CC89F: TUnixSystem::Exit(int, bool) (TUnixSystem.cxx:2153)
==6337==    by 0x5361A5B: TApplication::Terminate(int) (TApplication.cxx:1281)
==6337==    by 0x4E52AE2: TRint::Terminate(int) (TRint.cxx:686)
==6337==    by 0x4E5183E: TRint::Run(bool) (TRint.cxx:437)
==6337==  Address 0x11387c40 is 336 bytes inside a block of size 696 free'd
==6337==    at 0x4C2C171: operator delete(void*) (vg_replace_malloc.c:575)
==6337==    by 0x53BC849: TStorage::ObjectDealloc(void*) (TStorage.cxx:362)
==6337==    by 0x53A2A9A: TObject::operator delete(void*) (TObject.cxx:988)
==6337==    by 0x16353FA7: TTree::~TTree() (TTree.cxx:958)
==6337==    by 0x5416C48: TCollection::GarbageCollect(TObject*) (TCollection.cxx:734)
==6337==    by 0x541F3B6: TList::Delete(char const*) (TList.cxx:534)
==6337==    by 0x5419B66: THashList::Delete(char const*) (THashList.cxx:215)
==6337==    by 0x82D03C9: TDirectoryFile::Close(char const*) (TDirectoryFile.cxx:556)
==6337==    by 0x82E1049: TFile::Close(char const*) (TFile.cxx:942)
==6337==    by 0x52C411F: (anonymous namespace)::R__ListSlowClose(TList*) (TROOT.cxx:1092)
==6337==    by 0x52C4220: TROOT::CloseFiles() (TROOT.cxx:1113)
==6337==    by 0x52C48EF: TROOT::EndOfProcessCleanups() (TROOT.cxx:1190)
==6337==  Block was alloc'd at
==6337==    at 0x4C2B145: operator new(unsigned long) (vg_replace_malloc.c:333)
==6337==    by 0x53BC7DD: TStorage::ObjectAlloc(unsigned long) (TStorage.cxx:330)
==6337==    by 0x4E4F165: TObject::operator new(unsigned long) (TObject.h:152)
==6337==    by 0x162D5267: ROOT::new_TTree(void*) (G__Tree.cxx:3940)
==6337==    by 0x54669D2: TClass::New(TClass::ENewType, bool) const (TClass.cxx:4777)
==6337==    by 0x8329330: TKey::ReadObjectAny(TClass const*) (TKey.cxx:1058)
==6337==    by 0x82D1FB0: TDirectoryFile::GetObjectChecked(char const*, TClass const*) (TDirectoryFile.cxx:1044)
==6337==    by 0x16340AC4: void TDirectoryFile::GetObject<TTree>(char const*, TTree*&) (TDirectoryFile.h:80)
==6337==    by 0x405F27F: ???
==6337==    by 0x405E5F3: ???
==6337==    by 0x405E5BB: ???
==6337==    by 0x405E01D: ???
==6337==
==6337==