[avalanche] pass a fRelayProofs flag to the Peer struct

Summary:
This moves the responsibility of knowing whether avalanche is enabled to a scope that knows of the processor pointer. In a next diff, we will use the processor pointer to determine whether avalanche is enabled.
Depends on D16311

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16312