Run namespaces in their own thread pool

[eraserhd]

Closes #71

Except for setting up and tearing down "once fixtures", namespace threads are parked, meaning these cores aren't available for running tests. Additionally, when there are less threads than namespaces, the runner would deadlock when all test executor threads were being used by namespaces waiting for tests.

[eraserhd]

@weavejester I extracted a protocol to remove several conditionals instead of using delay. If that doesn't look better to you, I'll redo it.

Not entirely comfortable with the protocol name.

And, let me know if I should squash the commits.

Thanks! -Jason

[eraserhd]

That's all feedback addressed @weavejester

[weavejester]

Thanks for the update. Can you reformat the commit message to:

Run namespaces in their own thread pool

Except for setting up and tearing down "once fixtures", namespace
threads are parked, meaning these cores aren't available for running
tests. Additionally, when there are less threads than namespaces, the
runner would deadlock when all test executor threads were being used by
namespaces waiting for tests.

Closes #71.

So a wordwrap at the more standard 72 characters rather than 80 for the commit body.

Then I should be able to merge.

[eraserhd]

@weavejester done

[weavejester]

Manually merged.