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)
index, since the second item will have at most 1 entry.
It might make sense to just index timestamp, but we already have a composite
index which does this.
