Removed transaction logic from ChangeNotifier.
ChangeNotifier used a database transaction to wrapp changes.
This caused trouble because it was nested in an already open transaction,
which mediawiki doesn't like. I removed the transaction from ChangeNotifier:
There is no need for consistency between changes saved to the database.
ChangeNotifier contained some code for non-database transactions (or rather,
buffer-and-flush logic). This was unused and confusingly named, so I removed
it for now.