[avalanche] Sort txs to poll by modified fee rate

Summary: Then fallback to entry id in case of equality. Also manage a sort by txid if the txs are not in the mempool (which is currently unsupported by avalanche).

Test Plan:

ninja all check-all

Run the TSAN build.

Reviewers: #bitcoin_abc, PiRK, sdulfari

Reviewed By: #bitcoin_abc, PiRK, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13491