No API for service state

We already have core.hookenv.status_set and core.hookenv.status_get to set and get unit workload state. However, Juju status was a moving target when this was implemented and the 1.24 release also allows the leader to set the service state.

We could add an optional argument switching the existing functions to use the --service argument matching the multi-purpose hook tool, or a pair of new methods to keep things Pythonic.