[avalanche] Enable mining finalized transactions

Summary:
This require to only include the transactions that are still in the mempool.
Because the node keeps the radix tree untouched until the block is finalized, it would be possible for the node to generate a block template with confirmed transactions otherwise.

The feature is gated behind a dedicated flag for now. In the end this will be tied to preconsensus but this requires some tests to be updated first.

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18406