MULE-8018: Merge http connector code cleanup

Other than the package it's in (we can change that in the future) MuleMessageFactory isn't transport specific at all, it's for creating MuleMessage's from transport messages, something this new module also does, that fact that this isn't a transport doesn't means we shouldn't resuse what we can.  It's better to implement to an interface than just have a plain class, also this serves has an extension point, which although we might not expose in http module today maybe be useful in the future.