Revert "Improve performance of LDAPEntry.save according to f1ori's suggestions"

This reverts commit fa71aa00dc6dd75d72342ab9a4c12ea49ebf76b4.
Support deleting non-retrieved attributes from the server
Add test for creating an entry with an empty attribute
Raise LDAPNoSuchObjectError when fetching a non-existant attribute
Add multi_value boolean attributes to attribute mixins
Raise an error if an invalid LDAP attribute name is set
Move CFFI definitions from connection to a separate module
Implement LDAPEntry.set_password