Speed up retrying of failed external link checks

We retry external link checks in a few situations (e.g., to see if the server supports `GET` but not `HEAD`). But we were performing these retries in a blocking manner. We now put the retries into the `Typhoeus::Hydra` queue just like all the other requests, so they can be performed in parallel. This speeds up checking 62 links on a server that does not support `HEAD` from 20 seconds to 4 seconds on my machine.