[avalanche] Activate automated peer discovery
This diff makes the nodes to be added upon reception of a avahello
message. This feature is hidden behind a dedicated flag so one can
choose to only bind manually submitted nodes.
Depends on D9703.
ninja all check-all
Reviewers: #bitcoin_abc, PiRK, baby636
Reviewed By: #bitcoin_abc, PiRK
Maniphest Tasks: T1634
Differential Revision: https://reviews.bitcoinabc.org/D9739