Process unconfirmed_transactions by priority_fee and oldest timestamp

[vicsn]

Motivation

Following EIP-1559 and common decency, this PR ensures we process transactions by priority_fee. If the priority_fee is the same, we order by timestamp (in nanoseconds), and otherwise randomly by transaction_id.

It may be narrowly individually rational for validators to only sort by priority_fee and ignore the timestamp/id, but "It is recommended that transactions with the same priority fee be sorted by time the transaction was received" - https://eips.ethereum.org/EIPS/eip-1559 . This improves the performance and security of the whole network, which also matters for validators.

Test Plan

Separated the queue insertion and popping functions so they can be tested.