Removed transaction logic from ChangeNotifier.

Authored by daniel.


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.

backporting I014274289bb8bab34e25d8c3983816ce87296d2c

Change-Id: I9c0ce3f37e67094ba11d54d33d2d4bedacafd0e0