Abishopric/replicator

@mpuncel @petertseng this is the first set of commits that implement the replicator, which acts as a simple rollout mechanism for a single pod to multiple nodes. This is not ready to be merged.
I think I cargo culted a bit hard in the Allocation package - `MasterNode` and `Predicate` both don't have good uses yet. Will add comments for intended use.