Adjust on_job_posting to not hold the lock while investigating

To avoid issues when investigating jobs in one thread and having
a dispatcher thread get locked on that same investigation lock avoid
this entirely by not holding onto that lock.

This also adds a small cleanup around the notifier thread pool and
ensures that it actually exists in a useable state (and handles the
exceptions that arise when it doesn't) on submission.

Change-Id: I12e71f029726ec54ec0aadbdf71cc3c7a959f047
Make it so that the import works for older versions of kombu

The import location for message was different in older versions
of kombu, so when using it for mocking we need to take into
account the older location so that the test correctly works out
as it should (without import errors).

Fixes bug 1376330

Change-Id: Ifad489f0e15490168f993f48bfedfd553acd4a69
Merge "Adjust on_job_posting to not hold the lock while investigating"