[avalanche] refactor conditional processing in ProcessMessage

Summary:
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.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9338