[avalanche] Add a mempool isWorthPolling method for txs

Summary: This makes the code simpler and will allow for some optimizations in the future. There is no change in behavior.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, roqqit

Reviewed By: #bitcoin_abc, roqqit

Differential Revision: https://reviews.bitcoinabc.org/D17801