Rip out lift:when attribute from comet stuff.

We’re no longer using this to properly register comets on the
client.
Replace S.addCometAtEnd with S.addComet.

S.addComet takes a LiftCometActor instead of an Elem. It
replaces the private trackComet that simply added an entry
to S.requestCometVersions.

Additionally, this makes server-side actors and roundtrip
streaming promises work again.
Default S.findOrCreateComet to not call addComet.

The first pass made receiveUpdatesOnPage true by default;
on second thought, S.findOrCreateComet will by default not
add the comet to receive updates on the page. The Comet
snippet explicitly sets that parameter to true so that those
comets are registered to receive updates.

You can also separately receive updates by calling
S.addComet with the comet actor.