Use group updates when setting transaction status in clog.

Commit 0e141c0fbb introduced a mechanism
to reduce contention on ProcArrayLock by having a single process clear
XIDs in the procArray on behalf of multiple processes, reducing the
need to hand the lock around.  A previous attempt to introduce a similar
mechanism for CLogControlLock in ccce90b398
crashed and burned, but the design problem which resulted in those
failures is believed to have been corrected in this version.

Amit Kapila, with some cosmetic changes by me.  See the previous commit
message for additional credits.

Discussion: http://postgr.es/m/CAA4eK1KudxzgWhuywY_X=yeSAhJMT4DwCjroV5Ay60xaeB2Eew@mail.gmail.com

src/include/storage/proc.h

@@ -14,6 +14,7 @@
 #ifndef _PROC_H_
 #define _PROC_H_
 
+#include "access/clog.h"
 #include "access/xlogdefs.h"
 #include "lib/ilist.h"
 #include "storage/latch.h"
@@ -171,6 +172,17 @@ struct PGPROC
 
 	uint32		wait_event_info;	/* proc's wait information */
 
+	/* Support for group transaction status update. */
+	bool		clogGroupMember;	/* true, if member of clog group */
+	pg_atomic_uint32 clogGroupNext; /* next clog group member */
+	TransactionId clogGroupMemberXid;	/* transaction id of clog group member */
+	XidStatus	clogGroupMemberXidStatus;	/* transaction status of clog
+											 * group member */
+	int			clogGroupMemberPage;	/* clog page corresponding to
+										 * transaction id of clog group member */
+	XLogRecPtr	clogGroupMemberLsn; /* WAL location of commit record for clog
+									 * group member */
+
 	/* Per-backend LWLock.  Protects fields below (but not group fields). */
 	LWLock		backendLock;
 
@@ -242,6 +254,8 @@ typedef struct PROC_HDR
 	PGPROC	   *bgworkerFreeProcs;
 	/* First pgproc waiting for group XID clear */
 	pg_atomic_uint32 procArrayGroupFirst;
+	/* First pgproc waiting for group transaction status update */
+	pg_atomic_uint32 clogGroupFirst;
 	/* WALWriter process's latch */
 	Latch	   *walwriterLatch;
 	/* Checkpointer process's latch */