Increase gurantees of semantics of block/unblock/throwTo
Ensure threads that stopped on a throwTo because the target thread was in a 'block' state succeed at the next 'block'->'unblock' transition. Currently these stopped threads may never succeed making throwTo a possibly permanent hang.
The original paper, section 7.2, required the desired behavior.
More elaboration at http://haskell.org/haskellwiki/GHC/Concurrency/Flaws