[avalanche] Manage proof reception
Summary:
Upon proof reception, we should create a peer (no associated node yet),
relay the proof and update the tracker status.
Depends on D9660.
Ref T1611.
Test Plan:
With Clang and debug:
ninja all check-all
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Maniphest Tasks: T1611
Differential Revision: https://reviews.bitcoinabc.org/D9662