[plt-scheme] Cleanup on servlet timeout

I'm using the timeout dispatcher to stop responses which take too long
to compute. Normally when I throw an exception, the sqlite bindings
ensure that I release the database and I use dynamic-wind to release
my own locks I have claimed. However, when the request times out this
cleanup doesn't happen, I'm guessing because the thread is terminated
in a harsher way. Is there a way I could throw an exception in the
thread after a timeout instead of simply killing it?

Thanks,
    Henk