Async replication: refactor AsyncAllowedLag to time.Duration #512

Job	Run time
Build images	5s
test (GODOG_FEATURE=active_nodes.feature make test)	7m 2s
test (GODOG_FEATURE=async.feature make test)	4m 3s
test (GODOG_FEATURE=cascade_replicas.feature make test)	6m 42s
test (GODOG_FEATURE=CLI.feature make test)	3m 43s
test (GODOG_FEATURE=crash_recovery.feature make test)	4m 36s
test (GODOG_FEATURE=events_reenable.feature make test)	1m 53s
test (GODOG_FEATURE=failover.feature make test)	12m 3s
test (GODOG_FEATURE=free_space.feature make test)	5m 54s
test (GODOG_FEATURE=host_discovery.feature make test)	3m 13s
test (GODOG_FEATURE=host_management.feature make test)	2m 51s
test (GODOG_FEATURE=maintenance.feature make test)	4m 22s
test (GODOG_FEATURE=offline_mode.feature make test)	2m 13s
test (GODOG_FEATURE=priority.feature make test)	5m 13s
test (GODOG_FEATURE=readonly_filesystem.feature make test)	1m 53s
test (GODOG_FEATURE=recovery.feature make test)	2m 53s
test (GODOG_FEATURE=repair.feature make test)	9m 35s
test (GODOG_FEATURE=repl_mon.feature make test)	3m 34s
test (GODOG_FEATURE=statefile.feature make test)	3m 50s
test (GODOG_FEATURE=switchover_from.feature make test)	9m 44s
test (GODOG_FEATURE=switchover_to.feature make test)	10m 7s
test (GODOG_FEATURE=zk_failure.feature make test)	6m 58s
test (GODOG_FEATURE=zk_maintenance.feature make test)	3m 52s
	1h 56m 19s

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