[avalanche] Make the BlockFinalized validation callback asynchronous

Summary: There is no reason the make it synchronous. Also avoid holding a lock when calling the callback for good measure.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: tobias_ruck, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15193