Heartbeat mechanism

[jam231]

Current mechanism (setting SO_KEEPALIVE via sockopt) can take up to 2h to detect invalid connection state.
I propose incorporating heartbeat messages into protocol and the following algorithm for handling dead connection discovery:

1. Set timer to fire each x seconds with heartbeat signal.
2. Handle this signal by:
   For every connection:
   1. If connection has set probably_alive then negate probably_alive and send heartbeat to that connection.
   2. Else remove the connection from pool
3. On receiving from connection answer for heartbeat set probably_alive on the connection.