Pass the source text for a parallel query to the workers.

With this change, you can see the query that a parallel worker is
executing in pg_stat_activity, and if the worker crashes you can
see what query it was executing when it crashed.

Rafia Sabih, reviewed by Kuntal Ghosh and Amit Kapila and slightly
revised by me.

src/include/nodes/execnodes.h

@@ -371,6 +371,7 @@ typedef struct EState
 	Snapshot	es_crosscheck_snapshot; /* crosscheck time qual for RI */
 	List	   *es_range_table; /* List of RangeTblEntry */
 	PlannedStmt *es_plannedstmt;	/* link to top of plan tree */
+	const char *es_sourceText;	/* Source text from QueryDesc */
 
 	JunkFilter *es_junkFilter;	/* top-level junk filter, if any */