eio: enforce proper lifecycle for all Efl.Io_Model and fix discovered lifecycle bugs.
This make sure that the object returned by children_slice_get are properly
destroyed when the refcount drop to only the parent holding a reference on
it. This make it clear that the user of the api can rely on efl_ref/efl_unref
to actually manage its use of the returned object.
Additionnaly we are cleaning up the created object that we are using to build our own
request inside the Efl.Io.Model and avoid internal leak.
Depends on D7864
Reviewers: felipealmeida, segfaultxavi, SanghyeonLee, zmike, bu5hm4n
Reviewed By: zmike
Maniphest Tasks: T7528
Differential Revision: https://phab.enlightenment.org/D7865