Implement simple by-hash for apt update

Implement simple by-hash for apt update

This implements a apt update schema that get the indexfiles by the
hash instead of the name. The rational is that updates to the archive
servers/mirrors are not atomic so the client may have the previous
version of the Release file when the server updates to a new
Release file and new Packages/Sources/Translations indexes. By
keeping the files around by their hash we can still get the previous
indexfile without a hashsum mismatch.

Enable with APT::Acquire::By-Hash=1