Make delete tags return value consistent

This patch changes the return value for deleting tags to include the
list of uuids which have been affected.  This is potentially somewhat
confusing, since you can't tell from the reply if the stream or the
tag has been deleted but I think it generally makes sense.

This also fixes the "has" operator to work on uuids -- formally,
"select * where has uuid" returned nothing, because the uuids are
stored separately from the metadata hstore.
Allow sets to work without a where-clause

Although it's potentially not what you wanted, this lets you write something like:

   set Metadata/Location/Building = 'Building 1'

instead of needing to do something like:

   set Metadata/Location/Building = 'Building 1' where has Path

Or something.