support async replication #405

Job	Run time
Build images	7s
test (GODOG_FEATURE=active_nodes.feature make test)	7m 13s
test (GODOG_FEATURE=async.feature make test)	3m 36s
test (GODOG_FEATURE=cascade_replicas.feature make test)	7m 9s
test (GODOG_FEATURE=CLI.feature make test)	4m 1s
test (GODOG_FEATURE=crash_recovery.feature make test)	4m 26s
test (GODOG_FEATURE=events_reenable.feature make test)	2m 8s
test (GODOG_FEATURE=failover.feature make test)	12m 20s
test (GODOG_FEATURE=free_space.feature make test)	6m 10s
test (GODOG_FEATURE=host_discovery.feature make test)	3m 16s
test (GODOG_FEATURE=host_management.feature make test)	3m 5s
test (GODOG_FEATURE=maintenance.feature make test)	4m 20s
test (GODOG_FEATURE=offline_mode.feature make test)	2m 14s
test (GODOG_FEATURE=priority.feature make test)	5m 20s
test (GODOG_FEATURE=readonly_filesystem.feature make test)	2m 10s
test (GODOG_FEATURE=recovery.feature make test)	3m 7s
test (GODOG_FEATURE=repair.feature make test)	9m 58s
test (GODOG_FEATURE=statefile.feature make test)	4m 14s
test (GODOG_FEATURE=switchover_from.feature make test)	10m 21s
test (GODOG_FEATURE=switchover_to.feature make test)	10m 37s
test (GODOG_FEATURE=zk_failure.feature make test)	7m 21s
test (GODOG_FEATURE=zk_maintenance.feature make test)	4m 8s
	1h 57m 21s

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