[avalanche] Rename the peer set tag proof_index => by_proofid for consistency

Summary:
This is more consistent with other multi index tags, and this will also make it possible to templatize the proof_index structure so it can be used to index something else than a peer. There is no change in behavior.

Ref T1854.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10520