snapshot scalability: Move delayChkpt from PGXACT to PGPROC.

The goal of separating hotly accessed per-backend data from PGPROC
into PGXACT is to make accesses fast (GetSnapshotData() in
particular). But delayChkpt is not actually accessed frequently; only
when starting a checkpoint. As it is frequently modified (multiple
times in the course of a single transaction), storing it in the same
cacheline as hotly accessed data unnecessarily dirties a contended
cacheline.

Therefore move delayChkpt to PGPROC.

This is part of a larger series of patches intending to improve
GetSnapshotData() scalability. It is committed and pushed separately,
as it is independently beneficial (small but measurable win, limited
by the other frequent modifications of PGXACT).

Author: Andres Freund
Reviewed-By: Robert Haas, Thomas Munro, David Rowley
Discussion: https://postgr.es/m/20200301083601.ews6hz5dduc3w2se@alap3.anarazel.de

src/include/storage/proc.h

@@ -142,6 +142,8 @@ struct PGPROC
 	LOCKMASK	heldLocks;		/* bitmask for lock types already held on this
 								 * lock object by this backend */
 
+	bool		delayChkpt;		/* true if this proc delays checkpoint start */
+
 	/*
 	 * Info to allow us to wait for synchronous replication, if needed.
 	 * waitLSN is InvalidXLogRecPtr if not waiting; set only by user backend.
@@ -232,8 +234,6 @@ typedef struct PGXACT
 
 	uint8		vacuumFlags;	/* vacuum-related flags, see above */
 	bool		overflowed;
-	bool		delayChkpt;		/* true if this proc delays checkpoint start;
-								 * previously called InCommit */
 
 	uint8		nxids;
 } PGXACT;