x-sendfile writer: delegate serving files to optimized servers

[benoitbryon]

See at least:

• http://wiki.nginx.org/XSendfile
• https://tn123.org/mod_xsendfile/

Implement a writer which takes advantage of this X-SENDFILE header:

• write files to disk (temporary files) (could be done by another "write to disk" writer)
• return HTTP response with X-SENDFILE header referencing the file on disk
• at least explicitely tell users that they have to perform this cleaning operation. Or propose some (naive) pruning system, so that server's disk isn't filled with temporary files. Or maybe there is an nginx option to delete the file once it has been sent.