passing through the factory in order not to use explicitly the type.

This fixes ROOT-9664

in ROOT6 there is only one kind of reflection information. For this
reason, also the neighbouring references to Reflex have been removed.

Only this method will wait configured time for completion wait function.
WebWindow::WaitFor() does not have timeout option

Instead of using static variables just configure gEnv->SetValue()
parameters directly. Adjust tutorial

Always use mutex to protect vector of canvases 

This method is thread safe

Fixes several issues with drawing and update of superimposed histograms

works

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