`TransactionPool` API uses `async_trait` #14366

Job	Run time
preflight / preflight	40s
preflight / ci-versions	54s
test-linux-stable (1/3, parity-large)	39m 28s
test-linux-stable-int	9m 50s
test-linux-stable (1/3, parity-oldlinux)	39m 50s
test-linux-stable-no-try-runtime (1/2)	39m 45s
test-linux-stable-runtime-benchmarks	30m 18s
test-linux-stable (2/3, parity-large)	29m 40s
test-linux-stable-no-try-runtime (2/2)	30m 57s
test-linux-stable (2/3, parity-oldlinux)	24m 31s
test-linux-stable (3/3, parity-large)	29m 38s
test-linux-stable (3/3, parity-oldlinux)	23m 48s
All tests passed	1s
	4h 59m 20s

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