* Use the `ConditionVariable` primitive to fix rece between stopped
receiver Thread and its wake signal.
* Race was only observable via synthetical benchmarks and explicit
sleeps between unlocking receiver mutex and Thread.stop
* Simplifies code a lot as thread must not be passed around anymore
* I dislike the global infection RSpec does. This commit is a first step
to get a rid of it.
* Also remove the need for `require 'spec_helper` in each spec file with
adjusting `.rspec`.
