property_change() method doesn't really belong in sched_algorithm.

sched_algorithm knows nothing about properties, so that class isn't really the
best place for the property_change() virtual method. Introduce intermediate
sched_algorithm_with_properties_base class, which introduces property_change_()
virtual method accepting fiber_properties*. Then the properties-specific
sched_algorithm_with_properties<PROPS> implementation calls property_change()
(no trailing underscore) with fiber_properties* cast to PROPS&. Thus the user-
coded sched_algorithm implementation can override property_change() and accept
PROPS& rather than the generic fiber_properties* pointer. But
fiber_properties::notify() -- which doesn't know its own PROPS subclass -- can
nonetheless call sched_algorithm_with_properties_base::property_change_().
Fix bug in fiber::properties() method.

fm_properties() doesn't accept a boost::intrusive_ptr. Pass raw worker_fiber*
instead.
Only notify() a sched_algorithm_with_properties if it is_ready().

While it's true that not every READY fiber is always in the scheduler's ready
queue -- there's a time window in which a READY fiber can be on the
fiber_manager's wait queue -- only READY fibers will ever be on the
scheduler's ready queue. So if (! fiber.is_ready()), there's no point in
bothering the sched_algorithm implementation with a change to its properties.
Introduce sched_algorithm_with_properties::awakened_props() method.

Every sched_algorithm_with_properties<PROPS> subclass awakened() call must
ensure that control reaches sched_algorithm_with_properties<PROPS>::awakened()
_before_ any logic in the subclass method attempts to access properties. This
turns out to be all too easy to forget.

So instead, advise subclasses to overrride new awakened_props() method. Base-
class method sets things up and then calls awakened_props(). Moreover, when
the compiler supports it, sched_algorithm_with_properties<PROPS>::awakened()
is now marked 'final' to remind subclass authors to override awakened_props()
instead.
Add priority.cpp example program.

This illustrates use of a sched_algorithm_with_properties<PROPS> subclass that
defines, and supports, a priority property. The example includes a (somewhat
contrived) case in which a fiber must be moved within the scheduler's ready
queue due to a priority change.