`fatxpool`: proper handling of priorities when mempool is full #10018

Job	Run time
preflight / preflight	40s
preflight / ci-versions	1m 0s
build (polkadot-availability-recovery, availability-recovery-regression-bench)	8m 27s
build (polkadot-availability-distribution, availability-distribution-regression-bench)	6m 15s
build (polkadot-node-core-approval-voting, approval-voting-regression-bench)	9m 14s
build (polkadot-statement-distribution, statement-distribution-regression-bench)	5m 11s
publish-benchmarks	1s
	30m 48s

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