Refactor: update `raft-kv-memstore` example to use storage v2 #2881

Job	Run time
cluster-benchmark (cluster_benchmark)	46s
external-stores (stores/rocksstore-v2)	5m 48s
lint	6m 55s
examples (stable, raft-kv-memstore)	2m 15s
openraft-feature-test (nightly)	51s
examples (stable, raft-kv-rocksdb)	8m 5s
openraft-feature-test (nightly, serde)	51s
openraft-test (stable, 0)	2m 54s
openraft-test-bench (nightly)	1m 5s
examples (nightly, raft-kv-memstore)	2m 11s
openraft-feature-test (nightly, single-term-leader)	50s
openraft-test (nightly, 30)	3m 22s
openraft-feature-test (nightly, single-term-leader,serde,singlethreaded)	50s
examples (nightly, raft-kv-rocksdb)	12m 20s
openraft-test (nightly, 0, single-term-leader)	3m 9s
stores (memstore)	44s
stores (rocksstore)	6m 13s
Build (nightly, bench,serde,bt,singlethreaded)	1m 5s
stores (rocksstore-compat07)	6m 42s
tests-feature-test (nightly)	2m 59s
stores (sledstore)	57s
tests-feature-test (nightly, single-term-leader)	2m 43s
tests-feature-test (nightly, loosen-follower-log-revert)	2m 45s
	1h 16m 20s

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