Auto merge of #6271 - alexcrichton:h2, r=ehuss
Enable HTTP/2 by default
This commit switches Cargo to using HTTP/2 by default. This is
controlled via the `http.multiplexing` configuration variable and has
been messaged out for testing [1] (although got very few responses).
There's been surprisingly little fallout from parallel downloads, so
let's see how this goes!
[1]: https://internals.rust-lang.org/t/testing-cargos-parallel-downloads/8466