RFE: Separate negative cache timeout

[bbockelm]

For a variety of reasons, it's possible that a cmsd has a transient issue which prevents it from responding to location request.

Correspondingly, we should have the ability to do a separate timeout for a negative response and a positive response. For example, we might want to have a 15 minute timeout for cache entries with no location but 8 hours for a cache entry with at least one server location.

It's OK if "negative response" here is defined to mean "all servers respond negatively"; no need to try to tackle the case of a subset of servers.