ws_server can deadlock

[Niederb]

It happens when there are many incoming transactions/Websockets, such as during the benchmark.
This can happen due to the lock over the connections in ws_server.rs.
If the lock is not release properly then new connections cannot be accepted due to the lock being needed in accept_connection(...)

The lock is not being released as on_ready(..) is not returning. Inside on_ready(..) it seems to be read_or_initialize_websocket(..) that is blocking. This is where I stopped digging for now.

[Niederb]

I also saw now that the number of threads is "stuck" at 17 after the deadlock whereas before it seems to be changing between 17 and 20.