Implement a samplerd singleton actor service

Now spawned under the `pikerd` tree as a singleton-daemon-actor we offer
a slew of new routines in support of this micro-service:

- `maybe_open_samplerd()` and `spawn_samplerd()` which provide the
  `._daemon.Services` integration to conduct service spawning.
- `open_sample_stream()` which is a client-side endpoint which does all
  the work of (lazily) starting the `samplerd` service (if dne) and
  registers shm buffers for update as well as connect a sample-index
  stream for iterator by the caller.
- `register_with_sampler()` which is the `samplerd`-side service task
  endpoint implementing all the shm buffer and index-stream registry
  details as well as logic to ensure a lone service task runs
  `Services.increment_ohlc_buffer()`; it increments at the shortest period
  registered which, for now, is the default 1s duration.

Further impl notes:
- fixes to `Services.broadcast()` to ensure broken streams get discarded
  gracefully.
- we use a `pikerd` side singleton mutex `trio.Lock()` to ensure
  one-and-only-one `samplerd` is ever spawned per `pikerd` actor tree.