After some deep logging improvements to many parts of `._runtime`,
I realized a silly detail where we are always waiting on any opened
`local_nursery: ActorNursery` to signal exit from
`Actor._stream_handler()` even in the case of being an implicitly opened
root actor (`open_root_actor()` wasn't called by user/app code) via
`._supervise.open_nursery()`..
So, to address this add a `ActorNursery._implicit_runtime_started: bool`
that can be set and then checked to avoid doing the unnecessary
`.exited.wait()` (and any subsequent warn logging on an exit timeout) in
that special but most common case XD
Matching with other subsys log format refinements, improve readability
and simplicity of the actor-nursery supervisory log msgs, including:
- simplify and/or remove any content that more or less duplicates msg
content found in emissions from lower-level primitives and sub-systems
(like `._runtime`, `_context`, `_portal` etc.).
- add a specific `._open_and_supervise_one_cancels_all_nursery()`
handler block for `ContextCancelled` to log with `.cancel()` level
noting that the case is a "remote cancellation".
- put the nursery-exit and actor-tree shutdown status into a single msg
in the `implicit_runtime` case.
fb55784798