[avalanche] register our proof after IBD

Summary:
Use the updatedBlockTip signal to schedule the proof registration at first new block after IBD is finished.
Add a boolean flag to Processor to indicate that this should be done.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9323