Asks near first, if not found, asks far.
If found in there, returns it, and inserts it into
near.
Specifically, it always obtains the entire directory closure from far and inserts it into near,
which is useful when far does not support accessing intermediate directories (but near does).
There is no negative cache.
Inserts and listings are not implemented for now.
Holds references to many different directory services, each with an associated priority.
Read requests try services sequentially, sorted by their priority, ascending.
Any error in a service bubbles up.
Write requests are not implemented.