Remove thread_affinity flag and access methods.

Specificaly, remove access methods in worker_fiber, fiber and this_fiber.

thread_affinity is not used by any present library code. It was intended for
use by workstealing user sched_algorithm implementations. The properties
mechanism is a better way to address scheduler-specific properties.
Re-add thread_affinity specific to workstealing_round_robin.

thread_affinity is a good example of a property relevant only to a particular
sched_algorithm implementation. In examples/cpp03/migration, introduce an
'affinity' subclass of fiber_properties with a thread_affinity data
member.

Derive workstealing_round_robin from sched_algorithm_with_properties<affinity>
and, as required by that base class, forward awakened() calls to base-class
awakened() method.

Reimplement workstealing_round_robin's queue from a std::deque to a "by hand"
intrusive singly-linked list so we can efficiently remove an arbitrary item.
Make steal() method, instead of always popping the last item, scan the list to
find the last item willing to migrate (! thread_affinity).

From examples/cpp03/migration/workstealing_round_robin.hpp, an example of a
user-supplied sched_algorithm implementation, remove all boost/fiber/detail
 #includes. These should no longer be needed.

Change sched_algorithm_with_properties::properties(worker_fiber*) method to
accept fiber_base* instead. The original signature was introduced when every
sched_algorithm implementation necessarily manipulated worker_fiber* pointers.
Now we're intentionally avoiding the need.

For the same reason, introduce a fiber_properties::back_ptr typedef so
subclasses can opaquely pass such pointers through their own constructor to
the base-class constructor.
Remove 'priority' for every fiber, and its support methods.

Priority is another property that's only relevant for future sched_algorithm
implementations. We don't even have an example yet. It's a good candidate for
moving to a specific fiber_properties subclass for that specific
sched_algorithm implementation.