support async replication #461

Job	Run time
Build images	4s
test (GODOG_FEATURE=active_nodes.feature make test)	6m 45s
test (GODOG_FEATURE=async.feature make test)	3m 35s
test (GODOG_FEATURE=cascade_replicas.feature make test)	6m 43s
test (GODOG_FEATURE=CLI.feature make test)	3m 46s
test (GODOG_FEATURE=crash_recovery.feature make test)	4m 45s
test (GODOG_FEATURE=events_reenable.feature make test)	2m 0s
test (GODOG_FEATURE=failover.feature make test)	12m 36s
test (GODOG_FEATURE=free_space.feature make test)	5m 51s
test (GODOG_FEATURE=host_discovery.feature make test)	3m 6s
test (GODOG_FEATURE=host_management.feature make test)	3m 4s
test (GODOG_FEATURE=maintenance.feature make test)	4m 9s
test (GODOG_FEATURE=offline_mode.feature make test)	2m 13s
test (GODOG_FEATURE=priority.feature make test)	5m 11s
test (GODOG_FEATURE=readonly_filesystem.feature make test)	1m 59s
test (GODOG_FEATURE=recovery.feature make test)	2m 56s
test (GODOG_FEATURE=repair.feature make test)	9m 47s
test (GODOG_FEATURE=statefile.feature make test)	3m 55s
test (GODOG_FEATURE=switchover_from.feature make test)	10m 0s
test (GODOG_FEATURE=switchover_to.feature make test)	10m 0s
test (GODOG_FEATURE=zk_failure.feature make test)	7m 24s
test (GODOG_FEATURE=zk_maintenance.feature make test)	4m 1s
	1h 53m 50s

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