This removes "side" and "msgnum" from the URLs, and puts them in a JSON
request body instead. The server now maintains a simple set of messages
for each channel-id, and isn't responsible for removing duplicates.
The client now fetches all messages, and just ignores everything it sent
itself. This removes the "reflection attack".
Deallocate now returns JSON, for consistency. DB and API use "phase" and
"body" instead of msgnum/message.
This changes the DB schema, so delete the DB before upgrading the server.
The main wormhole code is str (unicode in py3, bytes in py2). Most
everything else must be passed as bytes in both py2/py3.
Keep the internal "side" string as a str, to make it easier to merge
with other URL pieces.
