handle UnboundedBlockingMpscQueue spurious wake-ups

A spurious wake-up can be produced by the new UnblockOnMainActorTest which
triggers the assert(!mpscQueue.empty()) in UnboundedBlockingMpscQueue::get.

Those spurious wake-ups are possible because the push and wake-up pair in
UnboundedBlockingMpscQueue::put are not atomic.
The following sequence diagram demonstrates a spurious wake-up:

   T1          T2            Q
   .           .            { }
  put(e)       .            { }
 push 54-57    .            {e}
   .         get()          {e}
   .        consume e       { }
   .           .            { }
   .         get()          { }
   .         block          { }
 unblock       .            { }
   .           .            { }
   .         wakeup         { }
   .           .            { }
               X
       assert(!queue.Empty())

To deal with spurious wake-ups we recheck the wake-up condition (a non empty queue)
and block again if we find it empty.
We assume spurious wake-ups are rare because it was difficult to reproduce them
even with a dedicated Test (the new UnblockOnMainActorTest) therefore we declare
the empty queue branch as unlikely.

Fixes #4.