Improve tuplesort.c to support variable merge order. The original coding

with fixed merge order (fixed number of "tapes") was based on obsolete
assumptions, namely that tape drives are expensive.  Since our "tapes"
are really just a couple of buffers, we can have a lot of them given
adequate workspace.  This allows reduction of the number of merge passes
with consequent savings of I/O during large sorts.

Simon Riggs with some rework by Tom Lane