Allow vacuums to report oldestxmin

Allow VACUUM and Autovacuum to report the oldestxmin value they
used while cleaning tables, helping to make better sense out of
the other statistics we report in various cases.

src/backend/commands/vacuumlazy.c

@@ -380,10 +380,11 @@ lazy_vacuum_rel(Relation onerel, int options, VacuumParams *params,
 							 vacrelstats->pinskipped_pages,
 							 vacrelstats->frozenskipped_pages);
 			appendStringInfo(&buf,
-							 _("tuples: %.0f removed, %.0f remain, %.0f are dead but not yet removable\n"),
+							 _("tuples: %.0f removed, %.0f remain, %.0f are dead but not yet removable, oldest xmin: %u\n"),
 							 vacrelstats->tuples_deleted,
 							 vacrelstats->new_rel_tuples,
-							 vacrelstats->new_dead_tuples);
+							 vacrelstats->new_dead_tuples,
+							 OldestXmin);
 			appendStringInfo(&buf,
 						 _("buffer usage: %d hits, %d misses, %d dirtied\n"),
 							 VacuumPageHit,
@@ -1329,8 +1330,8 @@ lazy_scan_heap(Relation onerel, int options, LVRelStats *vacrelstats,
 	 */
 	initStringInfo(&buf);
 	appendStringInfo(&buf,
-					 _("%.0f dead row versions cannot be removed yet.\n"),
-					 nkeep);
+					 _("%.0f dead row versions cannot be removed yet, oldest xmin: %u\n"),
+					 nkeep, OldestXmin);
 	appendStringInfo(&buf, _("There were %.0f unused item pointers.\n"),
 					 nunused);
 	appendStringInfo(&buf, ngettext("Skipped %u page due to buffer pins.\n",