Require that transactions selected for priority mining are below the minimum relay feerate.
This is the feerate that is required for priority to be the determining factor when a txn
is allowed into the mempool by AcceptToMempoolWorker.
The existing behavior allows for transactions with sufficient fee, but high priority,
to take up space reserved for high priority transactions. This seems to be outside
the intended purpose, as those transactions can already be allocated space within
addPackageTxn. If there are not enough free transactions to use the reserved space,
addPackageTxn will naturally overflow into this section.
Additionally, this is part of a larger effort to relax the transaction selection
within the mining code.