Change: Remove Box from `Snapshot::snapshot` #4481

Job	Run time
test-crate-tests (stable, 0)	3m 6s
test-crate-tests (nightly, 30)	3m 15s
Build (nightly, bench,serde,bt,singlethreaded)	53s
openraft-test-bench (nightly)	1m 4s
test-crate-tests (nightly, 0, single-term-leader)	3m 1s
openraft-feature-test (nightly)	51s
cluster-benchmark (cluster_benchmark)	48s
openraft-feature-test (nightly, serde)	51s
openraft-feature-test (nightly, serde,singlethreaded)	57s
rt-monoio	43s
test-crate-openraft (stable)	46s
stores (stores/memstore)	56s
test-crate-openraft (nightly)	55s
tests-feature-test (nightly)	3m 4s
tests-feature-test (nightly, single-term-leader)	3m 3s
examples (stable, memstore)	51s
examples (stable, rocksstore)	5m 52s
lint	3m 52s
examples (stable, raft-kv-memstore)	2m 39s
examples (stable, raft-kv-memstore-grpc)	2m 11s
examples (stable, raft-kv-memstore-network-v2)	53s
examples (stable, raft-kv-memstore-opendal-snapshot-data)	1m 9s
examples (stable, raft-kv-memstore-singlethreaded)	1m 3s
examples (stable, raft-kv-rocksdb)	5m 58s
examples (nightly, memstore)	1m 3s
examples (nightly, rocksstore)	10m 27s
examples (nightly, raft-kv-memstore)	2m 20s
examples (nightly, raft-kv-memstore-grpc)	2m 2s
examples (nightly, raft-kv-memstore-network-v2)	1m 8s
examples (nightly, raft-kv-memstore-opendal-snapshot-data)	1m 30s
examples (nightly, raft-kv-memstore-singlethreaded)	1m 7s
examples (nightly, raft-kv-rocksdb)	6m 3s
	1h 14m 21s

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