We replaced common::Socket with TcpSock from socket-collection. Then some tests
started failing. Turned out that common::Socket did a little trick after every
write(): it used to always reregister socket for Readable events. So we try
to retain the same behavior with TcpSock as well by reregistering socket readiness
manually where needed.

TCP listener was never used in case of direct connections. Hence, remove it.

socket_collection::UdpSock buffers the packets itself so no need to do that from the app layer.
We just make sure to UdpSock::write_to(None) when socket becomes writable.

…invalid

Also, includes some refactoring related to message sending.