Expose more information about BasicDbStorage queries

The debug bar for mediawiki transports the queries to the frontend by
encoding them to json.  Because our queries contain binary values json's
friendly utf-8 only requirement is nice enough to turn the query string
into null.  This adds a little bit more information so we at least know
what table was being queried.

Change-Id: Ia40a9fe015f043528b876ab7b5e380ef8a661c76
Merge "Expose more information about BasicDbStorage queries"