[avalanche] Build a radix tree of finalized txs

Summary:
This diffs adds a radix tree to store the finalized transactions.
This is in a preliminary state for now and is only used for the
isfinaltransaction RPC, so it can be tested.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15279