Prevent possible freeze when starting cluster with multiple service members

I haven't been able to reproduce the prod problem but the "threads stack trace" where pointing on concurrent calls to `forceServiceCheck()` (synchronous Actor call) from two different path: `ZookeeperClusterManager.start()` and another from the ZK notification thread. Now only the call from `start()` is synchronous and all other calls to `forceServiceCheck()` are not.