[avalanche] create an avalanche log category and use it

Summary: I've moved the current logging output from BCLog::NET to BCLog::AVALANCHE and a couple inconditional cases (accepted and rejected) to it also.

Test Plan:

ninja all check check-functional

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9781