Draining attributes in structures

This is so that @cyli can work independently on implementing the algorithm while I work on filling these structures.
Right, but the desired config might have non-0.  We do a comparison in `_converge_lb_state` to make sure that the weight and everything are all the same, else we produce a change step.  The comparison would not be equivalent because in the desired state, this will be non-zero, because the user would like the servers to be put into draining for some time before they are deleted.

But in all real nodes, whether draining or not, this value is 0, because the `LBNode` objects are synthesized from an actual LB node, which doesn't know about the draining timeout (i.e. this isn't the `draining_since` param).