[HELIX-549] Rethrow ThreadDeath instead of discarding.
[HELIX-550] ZKHelixManager should shutdown GenericHelixController.
apply review comments
Merge branch 'HELIX-550' of https://github.com/atcurtis/helix into helix-0.6.x (resolves #11)
[HELIX-550] ZKHelixManager should shutdown GenericHelixController, minor fix