[avalanche] Don't override cooldown with old rounds

Summary: This was marked as FIXME since the initial version. This diff solves it by storing the last round that updated the value in the node so it only updates if this is a newer round.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D19757