Add shared tuplestores.

Add shared tuplestores.

SharedTuplestore allows multiple participants to write into it and
then read the tuples back from it in parallel.  Each reader receives
partial results.

For now it always uses disk files, but other buffering policies and
other kinds of scans (ie each reader receives complete results) may be
useful in future.

The upcoming parallel hash join feature will use this facility.

Author: Thomas Munro
Reviewed-By: Peter Geoghegan, Andres Freund, Robert Haas
Discussion: https://postgr.es/m/CAEepm=2W=cOkiZxcg6qiFQP-dHUe09aqTrEMM7yJDrHMhDv_RA@mail.gmail.com