Allow parallel workers to execute subplans.

This doesn't do anything to make Param nodes anything other than
parallel-restricted, so this only helps with uncorrelated subplans,
and it's not necessarily very cheap because each worker will run the
subplan separately (just as a Hash Join will build a separate copy of
the hash table in each participating process), but it's a first step
toward supporting cases that are more likely to help in practice, and
is occasionally useful on its own.

Amit Kapila, reviewed and tested by Rafia Sabih, Dilip Kumar, and
me.

Discussion: http://postgr.es/m/CAA4eK1+e8Z45D2n+rnDMDYsVEb5iW7jqaCH_tvPMYau=1Rru9w@mail.gmail.com

src/include/nodes/primnodes.h

@@ -677,6 +677,7 @@ typedef struct SubPlan
 	bool		unknownEqFalse; /* TRUE if it's okay to return FALSE when the
 								 * spec result is UNKNOWN; this allows much
 								 * simpler handling of null values */
+	bool		parallel_safe;	/* OK to use as part of parallel plan? */
 	/* Information for passing params into and out of the subselect: */
 	/* setParam and parParam are lists of integers (param IDs) */
 	List	   *setParam;		/* initplan subqueries have to set these