Threaded dispatcher updates for {crew}

[shikokuchuo]

Now that threaded dispatcher is merged, this is to track changes needed to enable it for {crew}.

Inviting @wlandau.

[wlandau]

To directly interact with the mirai dispatcher, crew uses daemons(), saisei(), and status(). As long as those functions behave the same way with new threaded dispatcher as the existing process dispatcher, I think I will at least be able to start testing crew with the former. IIRC the other functions listed at https://github.com/wlandau/crew/blob/efb1f9b190504ab297f6738adde74d857d5e7066/R/crew_package.R#L25-L26 interact with the dispatcher indirectly or not at all.