[avalanche] Use the block fitter to track the finalized txs statistics

Summary:
This will later be used to check if a transaction would fit a block, and implement a back pressure feature to avoid creating blocks from finalized txs that are invalid or polling txs that could not be added to the next block.

There is no change in behavior.

Depends on D17793.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17795