This patch enables creation of a space with a foreign key pointing
to the same space. Such spaces are marked with a `selfpin` flag in
the `space_cache_pin_list`. If a space is pinned only by selfpins,
it is allowed to be deleted from the space cache.

When an old space is replaced by the new one, it's self-pinned for the
second time, then all old pins are moved to the new space's pin list
(see `space_cache_repin_pinned`), then the first self-pin is removed
by `space_cleanup_constraints` of the old space.

Closes #6961

NO_DOC=bugfix