[avalanche] Poll for the transactions that didn't make it into the last block

Summary:
After a block is finalized we need to poll for the remaining mempool transactions.
In order to select which transaction to poll we use the legacy block template constructor.

Depends on D17929.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, roqqit

Reviewed By: #bitcoin_abc, roqqit

Subscribers: roqqit

Differential Revision: https://reviews.bitcoinabc.org/D17832