[avalanche] Rename the peer set tag proof_index => by_proofid for consistency
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.
ninja all check
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Maniphest Tasks: T1854
Differential Revision: https://reviews.bitcoinabc.org/D10520