Better way of working with mirrors

Some services have several mirrors, yet we only use the first one by default rather than doing something cunning based on their geographical distance, etc.

e.g. astroquery.vizier

The issue names astroquery.vizier but no file, test, or entry point. Start by locating how that service selects its first mirror, then clarify the mirror-selection strategy, geographic or availability inputs, and tests needed to show that the chosen mirror is appropriate.

Written by the indexing model from the issue text.