[avalanche] Use argsman instead of gArgs throughout processor

Summary:
This removes the global dependency on gArgs in the avalanche processor. argsman
is already available so there is no reason to not use it.

There is no change in behavior.

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11527