Stop including <isc/json.h> and <isc/xml.h> from <isc/mem.h>, <isc/socket.h> and <isc/task.h>

Only named binary needs to include with <isc/json.h> and <isc/xml.h>, but the current code enforces inclusion of <isc/json.h> everywhere because <foo>_renderjson() and <foo>_renderxml() methods use native datatypes.

Fixing this would allow us make the Makefiles much simpler.

One way of fixing it (and probably simplest) would be making the function arguments opaque (isc_json_t * and isc_xml_t * types).

The other way would be to move <foo>_renderjson() and <foo>_renderxml() function declarations to their respective headers (<isc/json.h> and <isc/xml.h>), but that would be sort of namespace violation.

The last method that's probably cleanest would be to have just one <foo>_render() method that would accept isc_render_t structure with appropriate function calls. There's a lot of code duplication between <foo>_renderxml() and <foo>_renderjson() methods anyway.