[avalanche] Remote proofs: only account for the stake once when a peer has several nodes

Summary: The current heuristic account for the stake for each node, so if several nodes belong to a single peer they are accounted several times. This is not in par with the polling selection algorithm, so let's update the heuristic so it gets closer.

Test Plan:

ninja check-avalanche-peermanager_tests

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14957