Medium: asterisk: don't try to shut down with "core stop now"

If an asterisk instance is thoroughly shot, the "core stop now"
command will block. Since we'd been doing this in the foreground,
this would mean we could run into a timeout and an ugly stop failure.

In addition, doing "core stop now" adds no real benefit over just
sending a SIGTERM, where sending the SIGTERM is asynchronous and
hence, will not block -- thus our safeguards have a chance to kick
in before the timeout expires, and we can actually avoid the stop
failure.

Thanks to Rob Thomas for pointing this out.