Such that we maintain that subsys in the actor-runtime repo (with
hopefully an extensive test suite XD).
Port deats,
- rewrite `open_service_mngr()` as a thin wrapper that delegates into
the new `tractor.hilevel.open_service_mngr()` but with maintenance of
the `Services` class-singleton for now.
- port `.service._daemon` usage to the new
`ServiceMngr.start_service_ctx()` a rename from
`.start_service_task()` which is now likely destined for the soon
supported `tractor.trionics.TaskMngr` nursery extension.
- ref the new `ServiceMngr.an: ActorNursery` instance var name.
Other,
- always enable the `tractor.pause_from_sync()` support via `greenback`
whenever `debug_mode` is set at `pikerd` init.
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