[Reviewer: Graeme][From: Alex] Homestead changes following HttpStack re-factor

Graeme, please can you review this pull request. It contains:

* Co-req homestead changes to work with the new `HttpStack` and `Diameter::Stack` changes
* UTs for the new `HttpStackUtils` classes
Good spot. I think there are two ways to go on this:

* Make `_waiters` the number of threads that are actually waiting, in which case I should decrement. 
* Make `_waiters` be the number of times that `arrive` has been called. 

The second bullet was what I initially intended but on reflection I prefer the former approach.