server: Cleanup external address discovery.

This current code that handles the external server address discovery is
rather difficult to follow because it is poorly named and interspersed
with other code that is not related.

This refactors the code to make it better separated and easier to
follow.  It also switches the main data structure that limits the number
of addresses to use an LRU instead.

While refactoring this, I noticed there is room for improvement in terms
of the logic as well.  However, in order to keep the changes easier to
review for correctness, this does not contain any notable overall logic
changes and limits the changes to refactors and cleanup.

Author: davecgh