Check fallback directory performance before including them in the list
One fallback directory is quite slow to provide consensuses (~30s). atagar suggested in legacy/trac#18177 (moved) that we check fallbacks' speeds before we add them to the list (and provided code!)
We need to add this to the updateFallbackDirs.py script before we generate the next fallback list.
(One drawback of checking times is that we'll get slightly different results from different connections. This shouldn't be an issue if we download one at a time, and re-time failed connections.)