Convert cache system to github.com/Kovensky/go-fscache

Convert cache system to github.com/Kovensky/go-fscache

Includes wrapper methods that keep the Cached timestamp more-or-less
synchronized with the cache. Internally assert that the objects that
are supposed to have embedded timestamps implement the right interface.

Changes intent map to use interface{}, as there's no reason for the
intents to have to be cacheable.