Remove execution stages (creating sort index, sorting for group, sorting for
order, sorting result, removing duplicates); all of execution will now simply
become "executing". Execution stages have not given out meaningful information
for many, many years as execution has been overlapping, and with iterators,
they overlap even more, so it is meaningless to say that we are doing one stage
or the other.

Change-Id: I6ee334207212dd6ab724f9d4bc1fe4ad5f29689b