[avalanche] Limit the finalized txs radix tree size so it always fits in the next block

Summary:
This delays adding the transactions that won't fit the next block. This is a step toward making use of the radix tree for the block template.
For now the txs that are delayed remain sitting there, this will be fixed in the next diff.

Depends on D17829.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK, roqqit

Reviewed By: #bitcoin_abc, PiRK, roqqit

Subscribers: roqqit, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17831