[avalanche] Manipulate peers via PeerId

Summary: This ensure we do not expose information from internal data structure and can propose a stable API.

Test Plan:

ninja all check-avalanche

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6787