Not sure it's **that** useful (yet) but in theory would allow avoiding
certain log level usage around transient RPC requests for discovery methods
(like `.register_actor()` and friends); can't hurt to be able to
introspect that last message for other future cases I'd imagine as well.
Adjust the calling code in `._runtime` to match; other spots are using
the `trio.Nursery.start()` schedule style and are fine as is.
Improve a bunch more log messages throughout a few mods mostly by going
to a "summary" single-emission style where possible/appropriate:
- in `._runtime` more "single summary" status style log emissions:
|_mk `Actor.load_modules()` render a single mod loaded summary.
|_use a summary `con_status: str` for `Actor._stream_handler()` conn
setup and an equiv (`con_teardown_status`) for connection teardowns.
|_similar thing in `Actor.wait_for_actor()`.
- generally more usage of `.msg.pretty_struct` apis throughout `._runtime`.
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