- Replaced fiber_control internal scheduler with a lock-free queue for
faster context switching and scheduling.
 - fiber_group now has a more convenient version of the launch()
  function if you want to lock affinity to a single worker.
 - Removed the deprecated WAIT_FOR_REPLY and REPLY_PACKET message flags
 in RPC.
 - Added a FLUSH_PACKET message flag in RPC which requests for an
 message queue flush as soon as possible. This message flag is also
 inheritted when replying to a regular request call.
 - Removed a bunch of unnecessary flushes to allow for better message
 combining when fibers are used.
 - General Perfomrmance improvements in async stuff by ~25% on early
 benchmarks.