Introduce Suspendable Requests to the Event MPM.

Using this basic framework, you can return SUSPENDED from an HTTP Handler,
and then register a callback that is invoked by the MPM at a later time.

This initial version only supports _timers_ as callbacks, but in the future I
would like to add things like wait for socket activity, on a socket specified by
the handler.

Once in a callback, It is then the responsibility of the callback fucntion 
to finish the HTTP Request handling, but this alows you to do cool things like 
a fully async proxy, COMET support, or even rate limiting.

To prove I'm not insane, I've inlcuded an example module, mod_dialup.

You can configure it like this:
<Location "/docs">
  ModemStandard "V.32"
</Location>

And for static files inside that path, you will be rate limited to V.32 speeds, 
aka 9.6 kilobits/second.

Does anyone besides Rüdiger read commit emails :-) ?

I know there are likely huge problems with this, but I would like to see how far
we can push the Event MPM, fig...