Given it's a fairly simple yet useful abstraction, it makes sense to
offer this sub-sys alongside the core `tractor` runtime lib.
Without going into extreme detail on the impl changes (it'll come in
the commit that moves to the other repo) here is the high level summary:
------ - ------
- rename `Services` -> `ServiceMngr` and use an factory `@acm`
to guarantee a single-instance-per-actor using a niche approach for a
singleton object using a default keyword-arg B)
- the mod level `open_service_mngr()` and `get_service_mngr()` are the
new allocation/access API.
- add a `ServiceMngr.start_service()` method which does the work of both
spawning a new subactor (for the daemon) and uses its portal to start
the mngr side supervision task.
- open actor/task nurseries inside the `@acm` allocator.
Adjust other dependent subsystems to match:
------ - ------
- use `open_service_mngr()` when first allocated in `open_pikerd()`.
- use `get_service_mngr()` instead of importing the class ref inside
`.service.maybe_spawn_daemon()`, `.brokers._daemon.spawn_brokerd()`
and `.data._sampling.spawn_samplerd()` using a `partial` to pack in
the endpoint ctx kwargs (unpacked inside `.start_service()` XD).
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