Wait for busy authorities / fallback dir servers

In 6c443e98, we made the following change when selecting directory mirrors from the consensus: Tweak the 9969 fix a little

If we have busy nodes and excluded nodes, then don't retry with the
excluded ones enabled.  Instead, wait for the busy ones to be nonbusy.

We should do the same thing when selecting hard-coded authorities / fallback dir servers.

I have a patch for this.