In `ProcessMessage`, when checking for avalanche message types, there
are additional checks made in the same `if` statement. This can cause
the `Unknown command...` log message to be printed at the end of
`ProcessMessage` when the message is not unknown.
This has the additional benefit of factoring some repeated checks.
Add also a missing test for `unsolicited-avaproof` discouragment to
p2p_invalid_messages.py