This allows the Wormhole setup path to be simpler: consistently doing a
claim() just before watch(), regardless of whether we allocated the
channelid (with get_code), or dictated it (with set_code or
from_serialized).
The websocket lives on a Resource of the main rendezvous web site, and
the websocket URL is derived from the main "relay_url", so there's no
extra port to allocate, and no extra service to shut down.
