Rework Comet snippet.

In particular, the snippet now exposes a
Comet.containerForCometActor method that can be used by
an arbitrary third party consumer to produce the container for
a given comet actor when a handle to the LiftCometActor
object already exists.

It is, of course, still possible to materialize a CometActor using
regular lift:comet snippet.
Track comets to add to LiftMerge using RequestVar.

Before, comet tags included a lift:when attribute that was
used to extract comet versions and such. We now track
these when S.findOrCreateComet is called, and in LiftMerge
we use this list instead of looking for the marker attribute.

Because all invocations of the Comet snippet go through
S.findOrCreateComet, these are automatically tracked;
however, things added via S.addCometAtEnd are currently
lost, and we’re not handling AJAX send-down yet.
Fix AJAX send-down to use S.requestCometVersions.

This as opposed to using the lift:when marker attributes in
S.cometAtEnd.