To prepare for queue items moving into ZooKeeper, stop storing the
NodeRequest and NodeSet objects on them. Instead, reference requests
by ID and consult ZK when necessary, and store only the info about
nodesets that the scheduler needs. The result are simple dicts
than can easily be serialized.
The deleteNodeRequest method is updated to accept IDs instead of
NodeRequest objects to minimize the number of times we need to
use a full NodeRequest object.
Change-Id: I3587a42eb5a151f41369385e482b7f36b1c41bf6
6dc1178fc3