Fix multi-table VACUUM VERBOSE accounting.

Per-backend global variables like VacuumPageHit are initialized once per
VACUUM command.  This was missed by commit 49c9d9fc, which unified
VACUUM VERBOSE and autovacuum logging.  As a result of that oversight,
incorrect values were shown when multiple relations were processed by a
single VACUUM VERBOSE command.

Relations that happened to be processed later on would show "buffer
usage:" values that incorrectly included buffer accesses made while
processing earlier unrelated relations.  The same accesses were counted
multiple times.

To fix, take initial values for the tracker variables at the start of
heap_vacuum_rel(), and report delta values later on.

src/backend/access/heap/vacuumlazy.c

@@ -325,9 +325,12 @@ heap_vacuum_rel(Relation rel, VacuumParams *params,
 				new_rel_allvisible;
 	PGRUsage	ru0;
 	TimestampTz starttime = 0;
-	PgStat_Counter startreadtime = 0;
+	PgStat_Counter startreadtime = 0,
-	PgStat_Counter startwritetime = 0;
+				   startwritetime = 0;
-	WalUsage	walusage_start = pgWalUsage;
+	WalUsage	startwalusage = pgWalUsage;
+	int64		StartPageHit = VacuumPageHit,
+				StartPageMiss = VacuumPageMiss,
+				StartPageDirty = VacuumPageDirty;
 	ErrorContextCallback errcallback;
 	char	  **indnames = NULL;
 
@@ -639,12 +642,15 @@ heap_vacuum_rel(Relation rel, VacuumParams *params,
 			StringInfoData buf;
 			char	   *msgfmt;
 			int32		diff;
+			int64		PageHitOp = VacuumPageHit - StartPageHit,
+						PageMissOp = VacuumPageMiss - StartPageMiss,
+						PageDirtyOp = VacuumPageDirty - StartPageDirty;
 			double		read_rate = 0,
 						write_rate = 0;
 
 			TimestampDifference(starttime, endtime, &secs_dur, &usecs_dur);
 			memset(&walusage, 0, sizeof(WalUsage));
-			WalUsageAccumDiff(&walusage, &pgWalUsage, &walusage_start);
+			WalUsageAccumDiff(&walusage, &pgWalUsage, &startwalusage);
 
 			initStringInfo(&buf);
 			if (verbose)
@@ -763,18 +769,18 @@ heap_vacuum_rel(Relation rel, VacuumParams *params,
 			}
 			if (secs_dur > 0 || usecs_dur > 0)
 			{
-				read_rate = (double) BLCKSZ * VacuumPageMiss / (1024 * 1024) /
+				read_rate = (double) BLCKSZ * PageMissOp / (1024 * 1024) /
 					(secs_dur + usecs_dur / 1000000.0);
-				write_rate = (double) BLCKSZ * VacuumPageDirty / (1024 * 1024) /
+				write_rate = (double) BLCKSZ * PageDirtyOp / (1024 * 1024) /
 					(secs_dur + usecs_dur / 1000000.0);
 			}
 			appendStringInfo(&buf, _("avg read rate: %.3f MB/s, avg write rate: %.3f MB/s\n"),
 							 read_rate, write_rate);
 			appendStringInfo(&buf,
 							 _("buffer usage: %lld hits, %lld misses, %lld dirtied\n"),
-							 (long long) VacuumPageHit,
+							 (long long) PageHitOp,
-							 (long long) VacuumPageMiss,
+							 (long long) PageMissOp,
-							 (long long) VacuumPageDirty);
+							 (long long) PageDirtyOp);
 			appendStringInfo(&buf,
 							 _("WAL usage: %lld records, %lld full page images, %llu bytes\n"),
 							 (long long) walusage.wal_records,