Optimize how updates are done in _save() method

[birinsanchez]

django-ldapdb library always issues an ldap.MOD_REPLACE request to the LDAP server when saving an updated LDAP entry. This can cause big delays in some cases.

Imagine you have a model representing a LDAP group with a field of type ListField. That field is used to store the group members. If your group has thousands of members, when you add/remove a member django-ldapdb sends the whole list of members (thousands) to the LDAP server.

This PR sends ldap.MOD_REPLACE, ldap.MOD_DELETE or ldap.MOD_ADD depending on
how the value changes instead of always sending ldap.MOD_REPLACE.

[Natureshadow]

The logic is incomplete and wrong. The length has no meaning — jsut because the new attribute list has less entries does not imply that only entries were removed. Maybe 2 were removed and one new added, or even all removed and less new ones added?

You would have to compare the lists entry for entry. I do not think it's worth the effort, so closing. If you have a different opinion, please provide profiling that proves that the replace operation costs measurably more than tracing the changes.