[avalanche] Store the next possible conflict time for each peer

Summary:
This time will make it possible to implement cooldown features on proofs with utxos that conflict with an existing peer's proof.
This diff only adds the container and a method to update the time for a given peerid, which will be called in 2 places to set the cooldown:

• during a proof registration, to limit the conflict for the peer using a low value cooldown
• after a vote is finalized, to limit the peer replacement rate upon acceptation by the network.

Note that this time uses std::chrono::seconds and not TimePoint so it can be mocked in the tests.

Ref T1854.

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10717