support async replication #406

Job	Run time
Build images	11s
test (GODOG_FEATURE=active_nodes.feature make test)	6m 50s
test (GODOG_FEATURE=async.feature make test)	3m 28s
test (GODOG_FEATURE=cascade_replicas.feature make test)	6m 40s
test (GODOG_FEATURE=CLI.feature make test)	3m 50s
test (GODOG_FEATURE=crash_recovery.feature make test)	4m 26s
test (GODOG_FEATURE=events_reenable.feature make test)	2m 0s
test (GODOG_FEATURE=failover.feature make test)	11m 55s
test (GODOG_FEATURE=free_space.feature make test)	5m 41s
test (GODOG_FEATURE=host_discovery.feature make test)	3m 6s
test (GODOG_FEATURE=host_management.feature make test)	2m 57s
test (GODOG_FEATURE=maintenance.feature make test)	4m 19s
test (GODOG_FEATURE=offline_mode.feature make test)	2m 6s
test (GODOG_FEATURE=priority.feature make test)	5m 32s
test (GODOG_FEATURE=readonly_filesystem.feature make test)	2m 10s
test (GODOG_FEATURE=recovery.feature make test)	3m 18s
test (GODOG_FEATURE=repair.feature make test)	9m 26s
test (GODOG_FEATURE=statefile.feature make test)	3m 41s
test (GODOG_FEATURE=switchover_from.feature make test)	9m 54s
test (GODOG_FEATURE=switchover_to.feature make test)	10m 50s
test (GODOG_FEATURE=zk_failure.feature make test)	6m 58s
test (GODOG_FEATURE=zk_maintenance.feature make test)	3m 58s
	1h 53m 16s

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