Implement items retrieval and caching

- Sync token is set in EntriesFetchJob's mCollection's remoteRevision, which is then passed to CollectionModifyJob in slotItemsRetrieved()
- Changed CollectionCacheAttribute to EtebaseCache attribute, as it has to used for both collections and items
- Commented out item sync code in BaseHandler, which was used to handle conflict error (will see later)