Refactor: trait `AsyncOneshotSendExt` does not need `Unpin` #3421

Job	Run time
openraft-test (stable, 0)	3m 18s
cluster-benchmark (cluster_benchmark)	52s
openraft-test (nightly, 30)	3m 43s
openraft-test (nightly, 0, single-term-leader)	3m 31s
openraft-test-bench (nightly)	59s
stores (stores/memstore)	53s
Build (nightly, bench,serde,bt,singlethreaded)	51s
stores (stores/rocksstore)	6m 59s
stores (stores/sledstore)	1m 14s
examples (stable, memstore)	35s
lint	6m 48s
examples (stable, raft-kv-memstore)	2m 15s
examples (stable, raft-kv-memstore-generic-snapshot-data)	1m 45s
openraft-feature-test (nightly)	50s
examples (stable, raft-kv-memstore-opendal-snapshot-data)	1m 35s
tests-feature-test (nightly)	3m 9s
tests-feature-test (nightly, single-term-leader)	3m 21s
examples (stable, raft-kv-memstore-singlethreaded)	1m 0s
openraft-feature-test (nightly, serde)	54s
tests-feature-test (nightly, loosen-follower-log-revert)	3m 13s
examples (stable, raft-kv-rocksdb)	0s
openraft-feature-test (nightly, single-term-leader)	53s
examples (nightly, memstore)	55s
openraft-feature-test (nightly, single-term-leader,serde,singlethreaded)	54s
examples (nightly, raft-kv-memstore)	2m 56s
examples (nightly, raft-kv-memstore-generic-snapshot-data)	1m 30s
examples (nightly, raft-kv-memstore-opendal-snapshot-data)	2m 5s
examples (nightly, raft-kv-memstore-singlethreaded)	1m 43s
examples (nightly, raft-kv-rocksdb)	13m 1s
	1h 11m 42s

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