Such that the caller can be responsible for their own (nursery) scoping
as needed and, for the latter fn's case with
a `trio.Nursery.CancelStatus.encloses()` check to ensure the `tn` is
a valid parent-ish.
Some deats,
- in `gather_contexts()`, mv the `try/finally` outside the nursery block
to ensure we always do the `parent_exit`.
- for `maybe_open_context()` we do a naive task-tree hierarchy audit to
ensure the provided scope is not *too* child-ish (with what APIs `trio`
gives us, see above), OW go with the old approach of using the actor's
private service nursery.
Also,
* better report `trio.Cancelled` around the cache-miss `yield`
cases and ensure we **never** unmask triggering key-errors.
* report on any stale-state with the mutex in the `finally` block.