Test Celluloid's TaskThread runner for stability

Sidekiq uses Celluloid's TaskFiber by default.  MRI was recently discovered to have a Fiber bug which could lead to uncommitted transactions, open locks and other really odd behavior.  I'm unclear how to reproduce the problem but we should check TaskThread again to see if it's a stable replacement.

https://bugs.ruby-lang.org/issues/10540

/cc @halorgium