The internal zuul.nodepool.Nodepool.requests dictionary is used so
the scheduler can keep track of its requests. Since we will have
multiple schedulers emitting requests, we can't use that any more.
Remove any remaining uses of it.
The NodeRequest uid was only used to index that dictionary (and
was used to persist a request across resubmission). Since it isn't
needed any more, it is removed.
Change-Id: I7c82485d95979c6c9a246c3dc3954bae3c65ac13
b41f467340