[avalanche] Rename the ProofComparator

Summary: It compares by score, so let's call it ProofComparatorByScore. This makes it less confusing to add more comparators as needed. There is no change in behavior.

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11465