Added result deduping logic to HKWMentionsCreationStateMachine

- Added support for deduping within the results of a response and when appending
  a new response to existing response data

- Modified the asyncRetrieveEntitiesForKeyString completion block to take an
  additional boolean argument indicating if results should be deduped.

- Exposed a (NSString *)uniqueId method to HKWMentionsEntityProtocol which allows
  the caller to uniquely identify a mentions entity that may require more
  information than just the entityId