MULE-8018: Merge http connector code cleanup

Transports depend on MuleMessageFactory 100% agree, but MuleMessageFactory doesn't depend on anything else in transport and you change the package name (which we should do when we can) and it's a transport independent abstraction.  This isn't critical and neither does it affect functionalty but we should be using existing abstractions like this when possible.

Pablo K, up to you to make a call on this one..