Instead of starting every new thread for new operation - reuse same
thread which was started during first send operation.

Also take care that such dedicated threads correctly stopped when
WSHandler is destroyed

Only in such mode multithreading is allowed. 
In async mode one can meaningfully use send completion callback.
Default mode is synchronous (as before), which allows to use handler
only from main thread

Before one could submit as many operation as one wants to the websocket.
In case of longpoll socket this lead to extra queue, which was not
controlled by the user. 

Now each send operation is confirmed by callback. Only after such
callback next send operation via websocket is allowed. Queue is
organized externally (like in TWebWindow). 

For the longpoll engine this simplify internal logic and avoids queueing

Now no queues are allowed, all operation are confirmed by WS handler

Now one can directly register WS handlers to THttpServer
If allowed, handlers can be precessed directly in the civetweb threads
without involving of THttpServer thread at all. 
This allows to run many connections for many different web widgets
absolutely independent from each other - there are no one bottleneck in
http thread (or any other main thread)

In that folder all objects are registered, used by THttpServer
This allows to avoid all kind of global lists and exclude interfereance
between different instances of THttpServer. 
Such feature of main importance for TWebWindow(s), which should be
possible to run from many threads without interfearing with gROOT

To avoid 100% of CPU usage, made regulat short sleeps for 1ms

Should be used only with default sniffer, which does not have access to
any global lists

web socket engine should support this (default off) and
web socket handler have to allow this

When using builtin_openssl=ON, CMake erroneously exports the builtin
static libraries in ROOTConfig-targets.cmake, which causes problems with
dependent projects. In order to avoid this, we need Net and RHTTP to use
SSL as a private dependency. Since CMake requires target_link_libraries()
to either be all-plain (as before) or all-keyword (required to allow
PRIVATE linking), we need to move the build system to use all-keyword
linking only.

Fixes: ROOT-9532

One should always enable _GNU_SOURCE to use extra functionality from
glibc

THttpServer::SetTerminate() should only be used when really terminating
application. In such mode connections and websockets not cleaned up
properly

Signed-off-by: Olivier Couet <olivier.couet@gmail.com>

target

Now all these classes defined in http library internally

Was used to implement text-based CEF messaging, which is not efficient

Set ownership and websocket ID directly in that method

It is small sub-class, which allows to start http server like: 

    new THttpServer("https:4567?ssl_cert=certificate.pem");

Certificate should be provided in any case

If any SSL-related defines appeared in civetweb (even which says - do
not load SSL), civetweb starts to include ssl headers

Seems to be, Windows has a problem when many definitions combined
together

dependencies