Namely transferring the `Actor` peer-`Channel` tracking attrs,
- `._peers` which maps the uids to client channels (with duplicates
apparently..)
- the `._peer_connected: dict[tuple[str, str], trio.Event]` child-peer
syncing table mostly used by parent actors to wait on sub's to connect
back during spawn.
- the `._no_more_peers = trio.Event()` level triggered state signal.
Further we move over with some minor reworks,
- `.wait_for_peer()` verbatim (adjusting all dependants).
- factor the no-more-peers shielded wait branch-block out of
the end of `async_main()` into 2 new server meths,
* `.has_peers()` with optional chan-connected checking flag.
* `.wait_for_no_more_peers()` which *just* does the
maybe-shielded `._no_more_peers.wait()`
112ed27cda