Extend PgStat_HashKey.objid from 4 to 8 bytes

This opens the possibility to define keys for more types of statistics
kinds in PgStat_HashKey, the first case being 8-byte query IDs for
statistics like pg_stat_statements.

This increases the size of PgStat_HashKey from 12 to 16 bytes, while
PgStatShared_HashEntry, entry stored in the dshash for pgstats, keeps
the same size due to alignment.

xl_xact_stats_item, that tracks the stats items to drop in commit WAL
records, is increased from 12 to 16 bytes.  Note that individual chunks
in commit WAL records should be multiples of sizeof(int), hence 8-byte
object IDs are stored as two uint32, based on a suggestion from Heikki
Linnakangas.

While on it, the field of PgStat_HashKey is renamed from "objoid" to
"objid", as for some stats kinds this field does not refer to OIDs but
just IDs, like for replication slot stats.

This commit bumps the following format variables:
- PGSTAT_FILE_FORMAT_ID, as PgStat_HashKey is written to the stats file
for non-serialized stats kinds in the dshash table.
- XLOG_PAGE_MAGIC for the changes in xl_xact_stats_item.
- Catalog version, for the SQL function pg_stat_have_stats().

Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/ZsvTS9EW79Up8I62@paquier.xyz

src/include/access/xact.h

@@ -283,7 +283,13 @@ typedef struct xl_xact_stats_item
 {
 	int			kind;
 	Oid			dboid;
-	Oid			objoid;
+
+	/*
+	 * This stores the value of PgStat_HashKey.objid as two uint32 as all the
+	 * fields of xl_xact_xinfo should be multiples of size(int).
+	 */
+	uint32		objid_lo;
+	uint32		objid_hi;
 } xl_xact_stats_item;
 
 typedef struct xl_xact_stats_items

src/include/access/xlog_internal.h

@@ -31,7 +31,7 @@
 /*
  * Each page of XLOG file has a header like this:
  */
-#define XLOG_PAGE_MAGIC 0xD116	/* can be used as WAL version indicator */
+#define XLOG_PAGE_MAGIC 0xD117	/* can be used as WAL version indicator */
 
 typedef struct XLogPageHeaderData
 {