Implement timeout for server-side web sockets

[asvetlov]

Constructor parameter.
Should be None (disabled) by default.
If there is no activity from client-side peer for timeout seconds -- terminate the connection.

[asvetlov]

Write a doc describing why TCP transport has no guarantees for delivering socket disconnection event and why sending client-side pings (by ping frame or explicit message) is crucial for making connection reliable.

[fafhrd91]

implemented in master

[lock]

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