`fatxpool`: proper handling of priorities when mempool is full #7088

Job	Run time
preflight / preflight	43s
preflight / ci-versions	59s
test-frame-ui	9m 19s
test-full-crypto-feature	2m 55s
cargo-check-all-crate-macos	11m 28s
cargo-check-benches (master)	9m 21s
cargo-check-benches (current)	9m 20s
cargo-check-each-crate (1)	9m 21s
cargo-check-each-crate (2)	9m 16s
cargo-check-each-crate (3)	9m 14s
test-node-metrics	9m 21s
cargo-check-each-crate (4)	9m 15s
cargo-check-each-crate (5)	9m 15s
cargo-check-each-crate (6)	9m 13s
cargo-check-each-crate (7)	9m 1s
test-frame-examples-compile-to-wasm	3m 12s
test-deterministic-wasm	1s
node-bench-regression-guard	1s
check-tracing	1s
All test misc tests passed	1s
check-metadata-hash	1s
	2h 1m 18s

Provide feedback