introduce waker and wait_queue abstractions. fixes #251, #259

[dmitryikh]

introduce waker and wait_queue abstractions. fixes #251, #259

* remove context::wait_hook_ and context::twstatus in flavor to waker_epoch_ and waker class
* this avoids data races in case of wait_until() operations, when the context
  could be timeouted and rescheduled on the other OS thread. In this case could
  be data races with context::wait_hook_ and inconsistences context::twstatus
  states.
* using context::waker_epoch_ introduces mechanism when the old wakers become
  outdated and waker::wake() is just no op. This fixes data races explained in
  the previous point
* fibers waiting queue with timeouts and notification mechanisms are incapsulated into
  wait_queue class. This introduce simple abstraction level to be used in
  different synchronization primitives
* the same wait_queue is used in context::wait_queue_ to synchronize fiber::join() operations

I know there is a huge diff!
But this PR could be good for simplification and improved stability (correctness) of boost::fiber library.
Let's discuss it!

[olk]

verfy fine - ty Dmitry

[dmitryikh]

@olk, I don't expected that the PR will be so fast adopted. I expected there should be code review. But I don't familiar with boost.org rules..

[olk]

I've trust you - unfortunately I'm very busy and I've no time (but I looked at the changes in the PR).