Multilayer API Private maps (draft)

This page describes how windshaft should render maps with data from private tables

ideas

to have a new user in the database which provides access to data (the SQL API will not be able to use that user)
SQL, cartocss, interactivity and other params inside layergroup configuration are values that can be shared (are not private)
should be a share nothing architecture, that's it, there is not data stored anywhere with the map configuration (except the cartodb ui of course but that info is not used by windshaft)