Think how to invalidate materialized views

[zenitraM]

Coming from: https://cartodb.atlassian.net/browse/CDB-3931

We should invalidate materialized views by adding a DDL trigger on the REFRESH action of the view.. but then views can only be created over the SQL API.

Invalidating with the source table wouldn't work: you have to REFRESH the view for the changes to be applied.

Maybe adding a CDB_CartodbfyView which only adds the invalidation trigger?

cc @javisantana @rochoa @Kartones