field_attach_presave() is called too late

This is something I noticed while reviewing #2119467: Invoke entity CRUD hooks.

We call field_attach_presave() AFTER we've saved data. That's surely not right.

I think we need to clean up all of this part of the flag code:

- the _flagging_insert/update/delete() methods are all only called once, so I don't think they are necessary. They don't simplify the code (you have to jump around to follow the flow), and they have clearly led to bugs in the case of field_attach_presave().
- review the order in which all the various things are called, and check it makes sense in comparison with core entities
-- field_FOO API functions
-- entity CRUD hooks
-- Rules
-- our own hooks
- fold in the private methods _flag() and _unflag(). Again, they are only called from one place and just cause the flow the jump around. Also, _flag() should be converted to use drupal_write_record().