Runner now stores the override flag as a proc if one is
supplied. This proc is evaluated (in the context of the
calling app) each time override? is called
For sweatshop enqueue the tracer bits only if its allowed to run,
dequeue the tracer bits on only if they are attached
Tracer can take a proc for the override flag,
* it also delegates the config class (which delegates to
  the runner class) to check if the current override state
  is different from the supplied flag which could also be a proc.
Replaced Tracer's find_or_create with fetch.
No need for guard foo