[avalanche] Start polling when we park of block because of reorg

Summary: As per title.

Test Plan: Ran abc-p2p-avalanche test 25 times in a row to check tha it passes and isn't flacky.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5673