[Galene] Protocol rework

[Juliusz Chroboczek <jch@irif.fr>]

Hi,

I've just done some fairly major surgery to the protocol.  The code lives
in the branch "stable-id", I'll merge it into master later tonight.  It's
already deployed on galene.org, please let me know if I've broken anything.

Some background.  Galène's protocol is asynchronous: the state automata of
distinct flows are not synchronised between each other, which allows us to
perform multiple negotiations in parallel.  Obviously, this means that we
must take care not to mix up messages pertaining to distinct flows, so
each flow is identified by an "id", which is supposed globally unique.

Galène also performs flow stitching: when a user changes the parameter of
their flow, Galène will sometimes not attempt to renegotiate the flow, it
will simply replace the flow with a new one, negotiated from scratch.
This avoids tricky edge-cases related to renegotation, but requires the
protocol to carry information about which flow replaces which one.

We used to signal stitching by reusing the same id.  This made the code
somewhat insecure (we couldn't reliably identify an id collision, which
a malicious client could cause), and was racy — we'd sometimes mistake an
old answer for a recent one.

We now signal stiching explicitly, with a "replace" field in the "offer"
message.  This means that we can no longer use the flow id as the id of an
HTML element, so the client library now has a new "localId" which remains
stable across replacements.

This completes my plans for 0.3 — no new features before 0.3 is out.

-- Juliusz