Add helper "bound_v*_addrs" properties to http task

In case you need to get the bound adds by ip version..
Provide some hooks for graceful reactor shutdown

Provide some hooks to allow tasks that depend on twisted reactor task to
shutdown gracefully
Significantly change vservice shutdown procedure

Currently VService.stop() is a blocking call.  It signals to the tasks
that they should stop, and then blocks, waiting for them to stop.

Let's change that for now.  There are three functions now:

.stop() - Signal that the service and tasks should stop
._wait() - Wait (polling) for the tasks to shutdown
.join() - (unused) Call join() on all the threads.