[web] fix typo in doc string

[web] stop adding an Expires header with all responses As per RFC 1945 (HTTP 1.0): If the date given is equal to or earlier than the value of the Date header, the recipient must not cache the enclosed entity. As per RFC 7234 (HTTP 1.1 Caching): If a response includes a Cache-Control field with the max-age directive (Section 5.2.2.8), a recipient MUST ignore the Expires field. Bottom line, the Expires header is now handled wherever the Cache-Control is handled: inside cache managers.

[web] stop looking at a request's Cache-Control header As per RFC 7234 (HTTP 1.1 Caching): The "Cache-Control" header field is used to specify directives for caches along the request/response chain. CubicWeb is not a cache.

[fti] add cw_fti_index_rql_limit method Improves on and deprecate cw_fti_index_rql_queries: execute the rql directly, so that we don't need to compute the set of eids up-front, but can just keep track of the last seen entity. Use the new method in reindex_entities. Keep calling cw_fti_index_rql_queries if it has been redefined (e.g in cubes). Related to #3621392.

[fti] backout 166c6f7b1be4 * breaks tests * see next commit for a better solution

[py3k] Fix unicode added in commit 2350424585f8