rework long-polling server to be a valid EventSource

this is quite a bit of code change. I think I'll keep the code in the
talk on the level of the old version as that's much more readable
rework event-forwarder once more :-)

two things:

  1) We can add multiple handlers to one redis subscription connection.
     This way we don't need one connection per client, but one per
     channel, though we could even change that an multiplex in our own
     code.
  2) EventSource is subject to the same connection count limitation as
     is the default long-polling. This means that we have to play the
     waiting game in that case too.

Considering the amount of complexity for EventSource, combined with the
fact that it's still not supported in IE and the fact that it's also
subject to the connection count limit, I'm actually going to change my
recommendation in the talk to recommend to go with WebSockets or, if you
can't because you can't do SSL, then pure long-polling
remove redundant comments
add EventSource support to the frontend