Add a C API for parallel heap scans.

Using this API, one backend can set up a ParallelHeapScanDesc to
which multiple backends can then attach.  Each tuple in the relation
will be returned to exactly one of the scanning backends.  Only
forward scans are supported, and rescans must be carefully
coordinated.

This is not exposed to the planner or executor yet.

The original version of this code was written by me.  Amit Kapila
reviewed it, tested it, and improved it, including adding support for
synchronized scans, per review comments from Jeff Davis.  Extensive
testing of this and related patches was performed by Haribabu Kommi.
Final cleanup of this patch by me.