and replace tsleep(9) with msleep(9) which doesn't use a timeout. The
previously used timeout caused the event process to wake up ten times
per second on an idle system.
one_event() is now called with the topology lock held and it returns
with both the topology and event locks held when there are no more
events in the queue.
Reported by: mav, Marius Nünnerich
Reviewed by: freebsd-geom