Improve QlResponse, add proper caching support

Discard old no-cache behavior, adding support for caching compliant with
RFC 2616 (“HTTP/1.1”).

Improve wording to match RFC 2616 definitions, and explicitly mention RFC
2616 sections in parts of the code dealing with often-misused and
-misinterpreted header fields.

Renamed a few methods for consistency, and delete not-particularly-useful
QlResponse::send_last().