Use by-hash files when syncing content

[daviddavis]

Debian offers a feature called acquire-by-hash that prevents apt clients from downloading metadata files such as Release and Packages that don't match. This usually occurs when the repository is being published. These metadata files get published to a by-hash folder where they can be referenced by their hash digests. pulp_deb already supports publishing these files and ideally should support syncing using these files.