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The new Backend/TX split makes it a bit harder to test the validators,
but do so anyway.

Use a lower level type to abstract LDAP "transactions" (really just
batches of changes) and generate a set of ModifyRequest objects at
commit time. Change the API to let the caller manage the
transaction (TX object) lifetime.

This should make it easier to implement a SQL backend in the future if
necessary, even though LDAP knows no such thing as transactions.

As a result of a better low-level interface, reducing the boilerplate
LDAP code, the business logic in model.go should be quite more
readable.

This is a compiled version of the unboundid-ldap-server project,
with our own patches. All dependencies are included.

This includes a number of validators meant to support the creation of
new users and resources (for instance by checking for resource ID
uniqueness etc).

Also start adding some tests for the AccountService code.

The result is a functional server, altough there aren't nearly enough comments.