Test: add example of singlethreaded kv-store #2934

Job	Run time
external-stores (stores/rocksstore-v2)	5m 49s
cluster-benchmark (cluster_benchmark)	43s
examples (stable, raft-kv-memstore)	2m 9s
examples (stable, raft-kv-memstore-singlethreaded)	53s
lint	6m 37s
openraft-test (stable, 0)	2m 54s
examples (stable, raft-kv-rocksdb)	8m 13s
openraft-feature-test (nightly)	47s
examples (nightly, raft-kv-memstore)	2m 10s
openraft-test (nightly, 30)	3m 9s
openraft-feature-test (nightly, serde)	48s
openraft-test-bench (nightly)	54s
examples (nightly, raft-kv-memstore-singlethreaded)	1m 21s
openraft-test (nightly, 0, single-term-leader)	2m 58s
openraft-feature-test (nightly, single-term-leader)	47s
examples (nightly, raft-kv-rocksdb)	11m 53s
openraft-feature-test (nightly, single-term-leader,serde,singlethreaded)	48s
stores (memstore)	44s
Build (nightly, bench,serde,bt,singlethreaded)	50s
stores (rocksstore)	6m 3s
tests-feature-test (nightly)	2m 48s
stores (rocksstore-compat07)	6m 2s
tests-feature-test (nightly, single-term-leader)	2m 46s
stores (sledstore)	53s
tests-feature-test (nightly, loosen-follower-log-revert)	2m 39s
	1h 15m 38s

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