Provide much better wait information in pg_stat_activity.

When a process is waiting for a heavyweight lock, we will now indicate
the type of heavyweight lock for which it is waiting.  Also, you can
now see when a process is waiting for a lightweight lock - in which
case we will indicate the individual lock name or the tranche, as
appropriate - or for a buffer pin.

Amit Kapila, Ildus Kurbangaliev, reviewed by me.  Lots of helpful
discussion and suggestions by many others, including Alexander
Korotkov, Vladimir Borodin, and many others.

src/include/storage/lock.h

@@ -166,6 +166,8 @@ typedef enum LockTagType
 
 #define LOCKTAG_LAST_TYPE	LOCKTAG_ADVISORY
 
+extern const char *const LockTagTypeNames[];
+
 /*
  * The LOCKTAG struct is defined with malice aforethought to fit into 16
  * bytes with no padding.  Note that this would need adjustment if we were