Replace broken TCP connection management with a simple state machine.
Handle failed attempts properly with a timeout, detect and handle TCP
connection close and try to reconnect after that. Remove useless
'station_connected' flag.
Keep open messages saved even after the BMP session establishment,
so they can be used after BMP session flaps.
Use proper log messages for session events.
Most error handling code was was for cases that cannot happen,
or they would be code bugs (and should use ASSERT()). Keep error
handling for just for I/O errors, like in rest of BIRD.
Initial implementation of a basic subset of the BMP (BGP Monitoring
Protocol, RFC 7854) from Akamai team. Submitted for further review
and improvement.
