[avalanche] Add a facility to check if avalanche is enabled
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.
ninja all check-all
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Maniphest Tasks: T1611
Differential Revision: https://reviews.bitcoinabc.org/D9648