http: check longpoll websocket handling logic

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