[avalanche] Add a facility to check if avalanche is enabled

Summary:
This makes the code slightly better, but also makes it possible to
easily migrate away from gArgs or add other checks as needed.

Depends on D9647.

Ref T1611.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1611

Differential Revision: https://reviews.bitcoinabc.org/D9648