Auto merge of #4032 - alexcrichton:retry-500, r=matklad

Automatically retry HTTP requests returning 5xx

This commit implements auto-retry for downloading crates from crates.io whenever
a 5xx response is returned. This should help assist with automatic retries
whenever Cargo attempts to download directly from S3 but S3 returns a 500 error,
which is defined as "please retry again".

This logic may be a little eager to retry *all* 500 errors, but there's a
maximum cap on all retries regardless, so hopefully it doesn't result in too
many problems.

Closes #3962