[no merge] proposal to not cache tiles using non cartodbfied tables

[javisantana]

I was playing with FDW and I realized that we don't have a way to tell maps api to not cache tiles for some map. In a local table we can always attach a trigger so we update cdb_tablemedata but with a FDW is not possible.

The right way would be:

• using cdb_touchtablemedata ('fdw_table')
• not cache when a FDW is involved (not sure how to know this but there should be a catalog table with that information for sure)

anyways, I though wrapping the query in a function would work so I could use select * fn_that_query_my_fdw() but when I use that cache headers are set (and a wrong x-cache-channel)

This PR is a proposal to fix that. I think it's not right since a select 'POINT(1 1)'::geometry as the_geom_webmercator would not be cached (and it definitely can be) but it's better than caching something you don't know if cacheable

cc @rochoa

[rochoa]

The FDW thingy was addressed at #388. Although this was not 100% about FDW but about non-cartodbfied I'm closing it anyway.