- instead of using scheduling_algorithm::has_ready_fibers() use
a atomic counter as termination indication
- scheduling_algorithm::has_ready_fibers() returns true even if
only main-context and dispatcher-context are ready -> false indication
for work-sharing example
Instead, when the main fiber is passed to awakened(), stash it in a separate
slot and make pick_next() return it only when the shared queue is empty.
Update ready_fibers() to include a non-empty main_fiber slot.
