snapshot scalability: Move PGXACT->xmin back to PGPROC.

Now that xmin isn't needed for GetSnapshotData() anymore, it leads to
unnecessary cacheline ping-pong to have it in PGXACT, as it is updated
considerably more frequently than the other PGXACT members.

After the changes in dc7420c2c9, this is a very straight-forward change.

For highly concurrent, snapshot acquisition heavy, workloads this change alone
can significantly increase scalability. E.g. plain pgbench on a smaller 2
socket machine gains 1.07x for read-only pgbench, 1.22x for read-only pgbench
when submitting queries in batches of 100, and 2.85x for batches of 100
'SELECT';.  The latter numbers are obviously not to be expected in the
real-world, but micro-benchmark the snapshot computation
scalability (previously spending ~80% of the time in GetSnapshotData()).

Author: Andres Freund <andres@anarazel.de>
Reviewed-By: Robert Haas <robertmhaas@gmail.com>
Reviewed-By: Thomas Munro <thomas.munro@gmail.com>
Reviewed-By: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/20200301083601.ews6hz5dduc3w2se@alap3.anarazel.de

src/include/storage/proc.h

@@ -101,6 +101,11 @@ struct PGPROC
 
 	Latch		procLatch;		/* generic latch for process */
 
+	TransactionId xmin;			/* minimal running XID as it was when we were
+								 * starting our xact, excluding LAZY VACUUM:
+								 * vacuum must not remove tuples deleted by
+								 * xid >= xmin ! */
+
 	LocalTransactionId lxid;	/* local id of top-level transaction currently
 								 * being executed by this proc, if running;
 								 * else InvalidLocalTransactionId */
@@ -223,11 +228,6 @@ typedef struct PGXACT
 								 * executed by this proc, if running and XID
 								 * is assigned; else InvalidTransactionId */
 
-	TransactionId xmin;			/* minimal running XID as it was when we were
-								 * starting our xact, excluding LAZY VACUUM:
-								 * vacuum must not remove tuples deleted by
-								 * xid >= xmin ! */
-
 	uint8		vacuumFlags;	/* vacuum-related flags, see above */
 	bool		overflowed;