Multiple key support in the age-keyserver

It would be great to support multiple keys in the age-keyserver.

I use multiple keys for redundancy, and I would like to put them all up on https://keyserver.geomys.org

Start at the age-keyserver entry point and trace how a single key is currently stored and served. Review the behavior available at https://keyserver.geomys.org and determine the expected representation for multiple keys; done means users can publish and use all of their redundant keys consistently.

Written by the indexing model from the issue text.