Starting to draw a distinction between clean-close and abrupt-halt. At least,
if we're in the connected state, wormhole.close() should take its time and
free up server-side resources (nameplate/mailbox) right away, rather than
relying on GC/timeouts to release them.
It might be useful to make separate "clean" wormhole.close() and "abrupt"
wormhole.halt() API calls, except that really when would you ever call halt?
To be realistic, only one of two things will happen:
* connection happens normally, app finishes, calls "clean" close()
* app terminates suddenly, via exception or SIGINT
The problem with defining .close() is that I have to make it work sensibly
from any state, not just the one plausible "connected" state. Providing
.halt() requires defining its behavior from everywhere else.
