Entity tags: basic support in not modified filter. This includes handling of ETag headers (if present in a response) with basic support for If-Match, If-None-Match conditionals in not modified filter. Note that the "r->headers_out.last_modified_time == -1" check in the not modified filter is left as is intentionally. It's to prevent handling of If-* headers in case of proxy without cache (much like currently done with If-Modified-Since).