[Reviewer: Mike] Overload return codes

Mike, can you review this change to only return 503 when the current node is overloaded (or when a connection to the node times out). 504 is used when a downstream node is overloaded. 

I've also done some tidy up the hssconnection methods while testing this. 

This is part of the fix for #572 - cpp-common and homestead changes are needed too.