Refactor: update `raft-kv-memstore` example to use storage v2 #2880

Job	Run time
cluster-benchmark (cluster_benchmark)	46s
examples (stable, raft-kv-memstore)	2m 12s
examples (stable, raft-kv-rocksdb)	6m 30s
external-stores (stores/rocksstore-v2)	6m 6s
examples (nightly, raft-kv-memstore)	1m 45s
examples (nightly, raft-kv-rocksdb)	12m 12s
lint	7m 7s
openraft-feature-test (nightly)	50s
openraft-feature-test (nightly, serde)	53s
openraft-test (stable, 0)	2m 58s
openraft-feature-test (nightly, single-term-leader)	49s
openraft-test-bench (nightly)	58s
openraft-test (nightly, 30)	3m 18s
openraft-feature-test (nightly, single-term-leader,serde,singlethreaded)	49s
openraft-test (nightly, 0, single-term-leader)	2m 57s
Build (nightly, bench,serde,bt,singlethreaded)	49s
stores (memstore)	41s
tests-feature-test (nightly)	2m 44s
stores (rocksstore)	6m 12s
tests-feature-test (nightly, single-term-leader)	2m 26s
stores (rocksstore-compat07)	6m 6s
tests-feature-test (nightly, loosen-follower-log-revert)	2m 49s
stores (sledstore)	1m 3s
	1h 13m 0s

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