The generic observer delete was broken for GRDB, because we can't fetch the
uniqueId's of rowIds if the corresponding row was deleted. So instead, we
split the logic: Yap uses uniqueIds, GRDB uses row ids.
UIDatabaseObserver orchestrates notifying views when a transaction commits.
We back UIDatabaseObserver with a DatabaseRegionObserver
DatabaseRegionObservers are not notified if the transaction contains no changes:
https://github.com/groue/GRDB.swift/blob/af0ac15/GRDB/Core/DatabaseRegionObservation.swift#L119
However the more general TransactionObservers *are* notified even if the
transaction contains no changes. Nicely, GRDB does some work to ensure this is
true even in the event of a deferred transaction. See this comment on `databaseDidCommitEmptyDeferredTransaction`
da459386a0/GRDB/Core/TransactionObserver.swift (L444)
