[avalanche] Remember nodes pending a proof and bind them when available

Summary:
This diff adds a structure for remembering the nodes waiting for a
proof, and bind them as soon as this proof is known (i.e. a peer with
this proof is created).

Ref T1634.

Test Plan:

ninja all check-avalanche

Reviewers: #bitcoin_abc, PiRK, deadalnix

Reviewed By: #bitcoin_abc, PiRK, deadalnix

Subscribers: deadalnix

Maniphest Tasks: T1634

Differential Revision: https://reviews.bitcoinabc.org/D9703