Rsync endpoint for http://openresty.org/package/

@zenithalhourlyrate, as the maintainer of TUNA mirrors, reported a syncing issue in #200. Although this issue has been resolved, it would still be very helpful to provide an rsync endpoint for open-source mirror sites, as it could help:

• Reduce redundant requests and file transmission through network, and saves disk space of downstream, as many files (directories) are actually symlinks.
• Keep the integrity of synced packages better. I'm not sure whether openresty packages are behind CDN, but as far as I know the file listing by CDN may not be updated and thus HTTP-based syncing programs (lftp, etc.) cannot get a full list of packages and metadata files and bring users trouble.

I'm also the maintainer of one of the downstream mirrors of openresty (also we're users of openresty actually!). After a discussion with @zenithalhourlyrate, we think that I should open up a new issue to follow up the progress, as the original issue #200 is closed now.

Start by reviewing the repository's site and the prior discussion in issue #200 to understand how the package endpoint is currently served. Determine the appropriate way to provide an rsync endpoint for downstream mirrors; done means mirror maintainers can sync the OpenResty package tree reliably with reduced redundant transfers.