ingest/ledgerbackend: Create functional producer for BufferedStorageBackend #4269

Job	Run time
Test (and push) verify-range image	2m 26s
Integration tests (ubuntu-20.04, 1.21, 12, 21)	55m 14s
Integration tests (ubuntu-20.04, 1.21, 16, 21)	54m 41s
Integration tests (ubuntu-20.04, 1.22, 12, 21)	51m 23s
Integration tests (ubuntu-20.04, 1.22, 16, 21)	51m 21s
Integration tests (ubuntu-22.04, 1.21, 12, 21)	53m 30s
Integration tests (ubuntu-22.04, 1.21, 16, 21)	55m 28s
Integration tests (ubuntu-22.04, 1.22, 12, 21)	50m 33s
Integration tests (ubuntu-22.04, 1.22, 16, 21)	52m 2s
	7h 6m 38s

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