[avalanche] Rename getSuitableNodeToQuery => selectNode

Summary: The name is needlessly redundant, as piking suitable nodes to query is teh whole point of the PeerManager

Test Plan:

ninja all check-avalanche

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6845