Break up expensive queries, match index to quals

Break up expensive queries, match index to quals

First, we order browse queries over MBE by (sort_value, value) so we
want to match the unique index to that.

We're only going to use the first few rows of the cursors we build from
the back/forward MBE paging queries, and the embedded GROUP BY defeats
the planners desire to use an index for ordering the rows. So, instead,
we use a simpler core query and gather aggregate data as a secondary,
index-capable query for each MBE row.

Signed-off-by: Mike Rylander <mrylander@gmail.com>