Previous if the user did (implicitly or explicitly)
  - load first dictionary for a class (without a rootpcm)
  - access the TClass for that class
  - load a second dictionary (with a rootpcm) for the same class
  - Request the list of data members or the list of base class
    for that class.
then the pointer to either list was a nullptr (rather than a
pointer to the proper list as provided by the first dictionary).

Move locking in the caller which can mutate interpreter's state. Constify.

Revert "SetArguments calls reset arg list first. We don't want this when we pass a template parameter pack."

This introduced an issue. We really need to reset the elements every time we
send a signal.

Fixes the reported issues.

This reverts commit 2c3becf7.

Fix atexit function declarations for gcc and add test. Fix mangling issues for at_quick_exit and g++ headers.

Implement atexit override. Windows uses atexit is to drive invoke static destructors, and using atexit on other platforms will cause cling to crash.

Fix a segmentation violation when closing a detached TEveGedEditor, for example:
cd $(ROOTSYS)/root/tutorials/eve
root geom_atlas.C
<click on blue GLViewer [TGLSAViewer]>
<window de-attaches>
<click the window decoration to close window>
Root >
*** Break *** segmentation violation

This reverts commit f1538c4f304f470b8e24c4fc55f5724264427397.

Should fix a regression in the test suite.

Implement new naming in the relevant tutorials.
Residual fixes in the new class reorganization

This patch implenets the modifications required by the previous patch and
and makes the required internal adjustements

This patch realizes the reorganizational changes outlined in ROOT-8513.
In details:
   - Move TExecutor into core/base
   - The multi-thread Tree processor TTreeProcessor is renamed TTreeProcessorMT
   - The TTree porcessing part of TProcessExecutor is moved to a new
     class TTreeProcessorMP situated under tree/treeplayer (the interface
     is under ROOT) as TTreeProcessorMT. The methods are renamed 'Process' instead
     of ProcTree for symmetry.
   - The internal classes depending on libTree and libRIO (TPoolPlayer, TPoolProcessor)
     are also moved to tree/treeplayer and reorganized in such a way to remove code
     duplication. Their names are also changed to reflect their task.
     The new code is the new files TMPWorkerTree{.h,.cxx}.
   - The internal clas TPoolWorker is renamed TMPWorkerExecutor to reflect its task.