Range filter: single_range flag in request.

Range filter: single_range flag in request.

If set, it means that response body is going to be in more than one buffer,
hence only range requests with a single range should be honored.

The flag is now used by mp4 and cacheable upstream responses, thus allowing
range requests of mp4 files with start/end, as well as range processing
on a first request to a not-yet-cached files with proxy_cache.

Notably this makes it possible to play mp4 files (with proxy_cache, or with
mp4 module) on iOS devices, as byte-range support is required by Apple.