Refactor snapshot #2662

Job	Run time
cluster-benchmark (cluster_benchmark)	1m 8s
examples (stable, raft-kv-memstore)	3m 41s
external-stores (stores/rocksstore-v2)	10m 14s
examples (stable, raft-kv-rocksdb)	13m 48s
examples (nightly, raft-kv-memstore)	3m 30s
lint	10m 52s
examples (nightly, raft-kv-rocksdb)	24m 35s
openraft-feature-test (nightly)	1m 11s
openraft-feature-test (nightly, serde)	1m 16s
openraft-test (stable, 0)	4m 9s
openraft-test-bench (nightly)	1m 27s
openraft-feature-test (nightly, single-term-leader)	1m 12s
openraft-test (nightly, 30)	4m 33s
openraft-feature-test (nightly, single-term-leader,serde,singlethreaded)	1m 10s
openraft-test (nightly, 0, single-term-leader)	4m 29s
stores (memstore)	1m 16s
Build (nightly, bench,serde,bt,singlethreaded)	1m 25s
stores (rocksstore)	10m 23s
tests-feature-test (nightly)	3m 47s
stores (rocksstore-compat07)	13m 24s
tests-feature-test (nightly, single-term-leader)	4m 29s
stores (sledstore)	1m 14s
tests-feature-test (nightly, loosen-follower-log-revert)	3m 46s
	2h 6m 59s

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