Add a 'parallel_degree' reloption.

The code that estimates what parallel degree should be uesd for the
scan of a relation is currently rather stupid, so add a parallel_degree
reloption that can be used to override the planner's rather limited
judgement.

Julien Rouhaud, reviewed by David Rowley, James Sewell, Amit Kapila,
and me.  Some further hacking by me.