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[Eddie Kohler]

Yo Smenjie et al,

I made many changes and I'd like to run a test to see if they work. They
will hopefully get rid of the following bugs and issues,

(1) click-install and/or click-uninstall sometimes hangs -- was scheduler
    bug.

(2) TimedSource didn't work at user level -- hack was to give it tickets,
    fixed it properly (as well as analogous elements TimedSink, IPRewriter,
    ARPQuerier) by using a new schedule_immediately() function in
    Timer::element_timer().

(3) RatedSource had to keep rescheduling a timer 0ms in the future -- user
    level wait() now returns immediately if the work list is nonempty.

(4) TimedSource didn't work in kernel -- kernel driver formerly returned if
    nothing was scheduled. Now it does not return until the driver is
    stopped explicitly with please_stop_driver().

Click should probably run in a kernel thread even if HAVE_POLLING is not
defined. The reason is that the no-HAVE_POLLING code was already rotting,
and would only get worse. I implemented this change, but I'm not sure it
totally works. In particular, ToDevice might be broken when HAVE_POLLING is
not defined.

I also added the ability to hot-swap install a configuration. This is where
you install a new configuration, but if it fails, the old configuration
continues uninterrupted. If it initializes successfully, though, packets
from the old configuration are moved into the new Queues.

love,
ed