[avalanche] Move AvalancheNode to its own file

Summary:
As per title. This is a first step toward the integration of the PeerManager into AvalancheProcessor

Depends on D6828

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6830