Support parallel btree index builds.

 Support parallel btree index builds.
 
 To make this work, tuplesort.c and logtape.c must also support
 parallelism, so this patch adds that infrastructure and then applies
 it to the particular case of parallel btree index builds.  Testing
 to date shows that this can often be 2-3x faster than a serial
 index build.
 
 The model for deciding how many workers to use is fairly primitive
 at present, but it's better than not having the feature.  We can
 refine it as we get more experience.
 
 Peter Geoghegan with some help from Rushabh Lathia.  While Heikki
 Linnakangas is not an author of this patch, he wrote other patches
 without which this feature would not have been possible, and
 therefore the release notes should possibly credit him as an author
 of this feature.  Reviewed by Claudio Freire, Heikki Linnakangas,
 Thomas Munro, Tels, Amit Kapila, me.
 
 Discussion: http://postgr.es/m/CAM3SWZQKM=Pzc=CAHzRixKjp2eO5Q0Jg1SoFQqeXFQ647JiwqQ@mail.gmail.com
 Discussion: http://postgr.es/m/CAH2-Wz=AxWqDoVvGU7dq856S4r6sJAj6DBn7VMtigkB33N5eyg@mail.gmail.com