crates.io: Download changes

Like the rest of the Rust community, crates.io has been growing rapidly, with download and package counts increasing 2-3x year-on-year. This growth doesn't come without problems, and we have made some changes to download handling on crates.io to ensure we can keep providing crates for a long time to come. The Problem This growth has brought with it some challenges. The most significant of these is that all download requests currently go through the crates.io API, occasionally causing scaling iss...