add EventSubscribers to marathon.proto for sharing them with marathon instances
make prefix be customizable in MarathonStore
introduce modify(key)(S=>S) in MarathonStore.
Now, store is implemented in PersistenceStore as modify(key)(_=>newValue) .
Introduced SubscribersKeeperActor for accessing subscribers list stored in Zookeeper's state.  And, HttpEventActor now depends on SubscribersKeeperActor.

EventSubscriptionsResource depends on HttpCallbackSubscriptionService.  However, dependency of HttpCallbackSubscriptionService is initialized only when `--event_subscriber http_callback` is specified at startup.

So, normal @Injection occurs initialization exception of Guice.  Therefore, optional injection was used on HttpCallbackSubscriptionService so that injection skips when Guice couldn't instantiate HttpCallbackSubscriptionService.