Compatibility changes for purge module

Based on a solution proposed by @donquixote. When too much elements of an entity cachetags are going to be displayed on the X-Dropsolid-Purge-Tags header, nodes, media or taxonomy_terms, all the tags of that entity type is removed and changed to an "entity_type_list" (node_list, media_list or taxonomy_term_list). This reduces the amount of data displayed on the header and avoid the WSOD error.