[core] Replace Fog::Connection with stable version

Fog::Connection mixed in XML parsing via the `parser` argument which
wasn't much use for the majority of APIs using JSON.

This adds the deprecation warning and attempts to update providers to
the correct version of Connection that they need.

Either the cleaner `Fog::Core::Connection` or if reliant on the XML
parsing still `Fog::XML::SAXParserConnection`

The SAX parser will be moving to `fog/xml` fairly soon.