Async replication: refactor AsyncAllowedLag to time.Duration #513

Job	Run time
Build images	6s
test (GODOG_FEATURE=active_nodes.feature make test)	6m 57s
test (GODOG_FEATURE=async.feature make test)	3m 40s
test (GODOG_FEATURE=cascade_replicas.feature make test)	6m 47s
test (GODOG_FEATURE=CLI.feature make test)	3m 39s
test (GODOG_FEATURE=crash_recovery.feature make test)	4m 26s
test (GODOG_FEATURE=events_reenable.feature make test)	1m 59s
test (GODOG_FEATURE=failover.feature make test)	12m 3s
test (GODOG_FEATURE=free_space.feature make test)	5m 31s
test (GODOG_FEATURE=host_discovery.feature make test)	3m 2s
test (GODOG_FEATURE=host_management.feature make test)	2m 45s
test (GODOG_FEATURE=maintenance.feature make test)	4m 16s
test (GODOG_FEATURE=offline_mode.feature make test)	2m 6s
test (GODOG_FEATURE=priority.feature make test)	5m 27s
test (GODOG_FEATURE=readonly_filesystem.feature make test)	2m 10s
test (GODOG_FEATURE=recovery.feature make test)	2m 54s
test (GODOG_FEATURE=repair.feature make test)	9m 42s
test (GODOG_FEATURE=repl_mon.feature make test)	3m 25s
test (GODOG_FEATURE=statefile.feature make test)	3m 59s
test (GODOG_FEATURE=switchover_from.feature make test)	9m 54s
test (GODOG_FEATURE=switchover_to.feature make test)	10m 23s
test (GODOG_FEATURE=zk_failure.feature make test)	6m 47s
test (GODOG_FEATURE=zk_maintenance.feature make test)	4m 6s
	1h 56m 4s

Provide feedback