Reorganized the class hierary in com.swoop.data.mongo

* MongoCollectionConnector now has 2 subclassed versions for
  cacheable queries: memcached based and guava based.
* Using com.amazonaws.elasticache-java-cluster-client for the
  memcached client so that it could work with elasticache
  cache clusters, and autodiscovery.
* Introduced DatastoreStats interface so that cache statistics
  could be exposed.