[avalanche] Process AvalancheResponse and act based on the result.

Summary:
So far, we just processed repsonse and then diteched them away.

Because we do not poll for block in our chain, the park codepath is not tested. This will be the subject of the next patch.

Depends on D6548

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6559