Add LBFactory::beginMasterChanges() for doing DBO_TRX rounds

Add LBFactory::beginMasterChanges() for doing DBO_TRX rounds

This is in intended to replace the DataUpdate transaction round logic.
It could also be useful for doing transaction rounds in maintenance
scripts.

Also renamed $db => $conn in a few LB methods for consistency.

Change-Id: If21c2ba5e8bac48c250b96137279e7edaa8289f7