[avalanche] Extract proof reception logic out of the avaproof message handling

Summary: This will make it possible to reuse this logic when receiving proofs in batch from compact proofs messages. There is no change in behavior.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11520