Remove using= parameter from object methods.

This means if we load object from LDAP server we have to save it to that
same LDAP server.

In a practical sense, this was the case anyway.

Trying to make things too generic just overcomplicates everything.

This also means that the dn for an object will not change, unless rename
is called. To try and do otherwise is just so very very confusing.
Ensure alias set when object is created.
Set DN when creating object.
Update base_dn processing.
Delete using parameter.

Was required before 9f79c500bc971ad7fdd3dd6e4eb45d1b187d5f03, as we
couldn't be certain what LDAP server would be used.

In practise it was a complication, got me totally confused, and not sure
if I ever implemented it correctly.

Now we can just use self._alias, as this is guaranteed not to change for
the lifetime of the object.