Support new `by-hash` directive in the top-level Release and InRelease files.

[joliver]

The new by-hash directive completely eliminates race conditions, but requires all clients use Ubuntu 16.04 or later:

http://www.chiark.greenend.org.uk/~cjwatson/blog/no-more-hash-sum-mismatch-errors.html

[joliver]

Once supported, we should clean up older hashed index files only when they are not listed in the main package index and they are older than N days.

[joliver]

$ raptr upload --name="raptr" --category="public" --path="." --storage="us-west-1"

$ raptr link --name="raptr" --category="public" --version="0.1.19" --distribution="production" --storage="us-west-1"

$ gpg --armor --yes --detach-sign