`fatxpool`: proper handling of priorities when mempool is full #7818

Job	Run time
preflight	0s
build-linux-stable	0s
build-linux-stable-cumulus	0s
build-malus	0s
build-test-collators	0s
build-test-parachain	0s
prepare-bridges-zombienet-artifacts	0s
build-linux-substrate	0s
build-push-image-polkadot-parachain-debug	0s
build-push-image-polkadot-debug	0s
build-push-image-colander	0s
build-push-image-malus	0s
build-push-image-bridges-zombienet-tests	0s
build-push-image-test-parachain	0s
build-push-image-substrate-pr	0s
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