Add TIMING option to EXPLAIN, to allow eliminating of timing overhead.

Sometimes it may be useful to get actual row counts out of EXPLAIN
(ANALYZE) without paying the cost of timing every node entry/exit.
With this patch, you can say EXPLAIN (ANALYZE, TIMING OFF) to get that.

Tomas Vondra, reviewed by Eric Theise, with minor doc changes by me.

src/include/commands/explain.h

@@ -31,6 +31,7 @@ typedef struct ExplainState
 	bool		analyze;		/* print actual times */
 	bool		costs;			/* print costs */
 	bool		buffers;		/* print buffer usage */
+	bool		timing;			/* print timing */
 	ExplainFormat format;		/* output format */
 	/* other states */
 	PlannedStmt *pstmt;			/* top of plan */