[merkledb] add support for snapshots #8170

Job	Run time
Fuzz	14m 22s
Unit (macos-12)	8m 1s
e2e_post_etna	11m 14s
Unit (ubuntu-20.04)	4m 33s
Unit (ubuntu-22.04)	7m 10s
e2e_pre_etna	11m 13s
Protobuf Lint	10s
Unit (windows-2022)	11m 31s
Unit (custom-arm64-focal)	1s
e2e_existing_network	4m 26s
Unit (custom-arm64-jammy)	14m 19s
Lint	3m 18s
Upgrade	3m 11s
Up-to-date protobuf	34s
Up-to-date go.mod and go.sum	10s
Up-to-date mocks	2m 6s
Build Antithesis avalanchego images	8m 55s
Build Antithesis xsvm images	11m 54s
Image build	10m 42s
	2h 7m 50s

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