factor "shutdown" flow into one place

[meejah]

If the "nice shutdown" flow actually works well, factor it into FowlWormhole.close_wormhole (or similar). If it doesn't (or if it goes in magic-wormhole itself) then delete it.

[meejah]

Also, the "closing -> closed" dance isn't fully correct: the side that didn't initiate the closing flow never is satisfied and keeps waiting.