Rewrite the main loop for simplicity and perf

[dhleong]

This is a massive refactor that consolidates all the many ways we have of getting onto the main thread into a single Dispatcher abstraction. In particular:

• The main loop is no longer a loop around many tiny sleeps, and instead is exclusively a "wait" on a channel, which should overall reduce power usage since the vast majority of time in a MUD is probably waiting for something to happen (either output from the server, or input from the user).
• Connections are rewritten to be multi-threaded
• The "Connection" abstraction was renamed to "Transport," which better describes how it is used
• Jobs still exist as a means of supporting multi-threaded work, but they access the main thread through Dispatcher (including support for results!)
• Scripts (of course) also access the main thread via Dispatcher
• Main thread dispatches are handled in batches; if multiple come in at once, we consistently handle them all so long as we don't have to do a blocking read to get them, and the elapsed processing time does not exceed one "frame" of time.
• Complete removed the ApiManagerDelegate since we no longer need it; this solves the discussion around re-entrant scripting since we don't have to remove anything from app::State to respond to scripting methods.

May just be placebo, but things do feel a bit snappier now!

Future work might:

• Rewrite Telnet support using on Tokio's async TCP connections
• Refactor Dispatcher to receive an Enum so we can provide an explicit "There's a pending key" signal that shortcuts out of whatever it's doing
• Refactor Connections to not be Option on app::State; not strictly necessary (or very relevant to this PR) but also I don't think we need it, and that refactor would certainly simplify access to connections