Refactor: add examples/memstore to implement LogStore for other example applications #3137

Job	Run time
cluster-benchmark (cluster_benchmark)	47s
external-stores (stores/rocksstore-v2)	6m 11s
openraft-test (stable, 0)	3m 4s
openraft-test-bench (nightly)	55s
openraft-test (nightly, 30)	3m 25s
openraft-test (nightly, 0, single-term-leader)	3m 6s
stores (memstore)	49s
stores (rocksstore)	6m 13s
Build (nightly, bench,serde,bt,singlethreaded)	50s
stores (rocksstore-compat07)	6m 13s
examples (stable, memstore)	49s
stores (sledstore)	1m 1s
lint	6m 29s
examples (stable, raft-kv-memstore)	2m 14s
openraft-feature-test (nightly)	50s
examples (stable, raft-kv-memstore-generic-snapshot-data)	54s
tests-feature-test (nightly)	2m 51s
examples (stable, raft-kv-memstore-singlethreaded)	54s
tests-feature-test (nightly, single-term-leader)	2m 51s
examples (stable, raft-kv-rocksdb)	7m 52s
tests-feature-test (nightly, loosen-follower-log-revert)	2m 49s
examples (nightly, memstore)	52s
examples (nightly, raft-kv-memstore)	2m 9s
openraft-feature-test (nightly, serde)	49s
openraft-feature-test (nightly, single-term-leader)	50s
examples (nightly, raft-kv-memstore-generic-snapshot-data)	1m 16s
openraft-feature-test (nightly, single-term-leader,serde,singlethreaded)	51s
examples (nightly, raft-kv-memstore-singlethreaded)	1m 17s
examples (nightly, raft-kv-rocksdb)	12m 17s
	1h 21m 28s

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