[avalanche] Add the notion of Peer to the PeerManager

Summary:
An avalanche Peer can be one or more nodes, so that a large peer can instanciate several nodes to be able to serve more requests.

This allows to rework the compactions algorithm to make it much simpler.

Test Plan:

ninja check-avalanche

The update of the verify function should check that the Peer remain in a coherent state.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6818