In the future, both sides should expect to receive "transit" messages at
any time, and they will add to the list of hints that they should try.
For now, each side only sends a single transit message, before they send
the offer (sender) or answer (receiver).
This moves us slowly towards a file-transfer protocol that exchanges
multiple messages, with a single offer (sender->receiver) and
answer (receiver->sender), and one or more connection hint messages (in
either direction) that appear gradually over time as connection
providers come online.
At present the protocol still expects the whole hint list to be present
in the offer/answer message.
One downside is that we keep the wormhole channel allocated longer (we
have to finish the file transfer before we can deallocate it, which
could take a while for large files). Maybe we can fix this in the
future.
Also clean up test_scripts.PregeneratedCode:
* fetch results from both sides at the same time
* only check rc when using a subprocess, since the direct call doesn't
use rc=0 anymore
* no need to cancel the other side's Deferred when one errors
* provide more information if stderr was non-empty
These were split out to make the blocking- and twisted- based
implementations share some code, but now that we're down to just
Twisted, it's clearer to merge them back in.
