Changed TOAST relations to have relkind RELKIND_TOASTVALUE.

Special handling of TOAST relations during VACUUM. TOAST relations
are vacuumed while the lock on the master table is still active.
The ANALYZE flag doesn't propagate to their vacuuming because the
toaster access routines allways use index access ignoring stats, so
why compute them at all.

Protection of TOAST relations against normal INSERT/UPDATE/DELETE
while offering SELECT for debugging purposes.

Jan