CBLIS - Improve relationship cache invalidation

[pasin]

Relationship cache is currently invalidated by checking between CBLQueryEnumerator.lastSequence and database.lastSequence. As a result, the whole cache will be invalidated even though the changes to the database are not relevant to the cache. One way to improve this is to invalidate cache based on changes notified by the CBLDatabaseChangeNotification. Another way would be checking if the view is changed or not - this needs to call View.updateIndex first and check View.lastSequenceChangedAt to check. Note that View.updateIndex and View.lastSequenceChangedAtare all private.