Introduced SubscribersKeeperActor for accessing subscribers list stored in Zookeeper's state.  And, HttpEventActor now depends on SubscribersKeeperActor.
Optional injection on EventSubscriptionsResource for realizing conditional injection.

Api resources must be always published because http requests to marathon instance will be forwarded to the leader even when --event_subscriber was NOT specified at startup.

However, EventSubscriptionResource depends on HttpCallbackSubscriptionService. And,  dependencies of HttpCallbackSubscriptionService are initialized only when --event_subscriber http_callback is specified at startup. Therefore,  if the normal @Inject annotation was used,  Guice raises an initialization exception during instantiating HttpCallbackSubscriptionService. Optional injection can avoid this.