rework Python SchedulerClient

We need a better URI scheme for locating schedulers. Right now we conflate cluster name with cluster location (e.g. ablocal, localhost:1234, smf1-test)
SchedulerClient is also poorly factored, e.g. LocalSchedulerClient, ZookeeperSchedulerClient. Instead there should be separate local / zookeeper resolvers that then feed into a non-inherited SchedulerClient.
Also, most people should be interacting with SchedulerProxy. It's likely that SchedulerProxy/SchedulerClient should be merged together, and possibly the session acquisition should be plumbed into the AuroraClientAPI.