| 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). | ||
|---|---|---|
| .. | ||
| _inputs | ||
| conftest.py | ||
| test_accounting.py | ||
| test_cli.py | ||
| test_docker_services.py | ||
| test_ems.py | ||
| test_feeds.py | ||
| test_questrade.py | ||
| test_services.py | ||
| test_watchlists.py | ||