Improve memory space management in tuplesort and tuplestore.

The code originally just doubled the size of the tuple-pointer array so
long as that would fit in allowedMem.  This could result in failing to use
as much as half of allowedMem, if (as is typical) the last doubling attempt
didn't quite fit.  Worse, we might double the array size but be unable to
use most of the added slots, because there was no room left within the
allowedMem limit for tuples the slots should point to.  To fix, double only
so long as we've used less than half of allowedMem in total.  Then do one
more array enlargement, but scale it based on total memory consumption so
far.  This will work nicely as long as the average tuple size is reasonably
stable, and in any case should be better than the old method.

This change will result in large sort operations consuming a larger
fraction of work_mem than they typically did in the past.  The release
notes should mention that users may want to revisit their work_mem
settings, if they'd tuned those settings based on the old behavior of
sorting.

Jeff Janes, reviewed by Peter Geoghegan and Robert Haas