Remove remants of "snapshot too old"

Remove the 'whenTaken' and 'lsn' fields from SnapshotData. After the
removal of the "snapshot too old" feature, they were never set to a
non-zero value.

This largely reverts commit 3e2f3c2e42, which added the
OldestActiveSnapshot tracking, and the init_toast_snapshot()
function. That was only required for setting the 'whenTaken' and 'lsn'
fields. SnapshotToast is now a constant again, like SnapshotSelf and
SnapshotAny. I kept a thin get_toast_snapshot() wrapper around
SnapshotToast though, to check that you have a registered or active
snapshot. That's still a useful sanity check.

Reviewed-by: Nathan Bossart, Andres Freund, Tom Lane
Discussion: https://www.postgresql.org/message-id/cd4b4f8c-e63a-41c0-95f6-6e6cd9b83f6d@iki.fi

src/backend/access/heap/heaptoast.c

@@ -639,7 +639,6 @@ heap_fetch_toast_slice(Relation toastrel, Oid valueid, int32 attrsize,
 	int			endchunk;
 	int			num_indexes;
 	int			validIndex;
-	SnapshotData SnapshotToast;
 
 	/* Look for the valid index of toast relation */
 	validIndex = toast_open_indexes(toastrel,
@@ -685,9 +684,8 @@ heap_fetch_toast_slice(Relation toastrel, Oid valueid, int32 attrsize,
 	}
 
 	/* Prepare for scan */
-	init_toast_snapshot(&SnapshotToast);
 	toastscan = systable_beginscan_ordered(toastrel, toastidxs[validIndex],
-										   &SnapshotToast, nscankeys, toastkey);
+										   get_toast_snapshot(), nscankeys, toastkey);
 
 	/*
 	 * Read the chunks by index