Make the cache memory bounded, by using a LRU cache. Also split the
cache in a positive and negative one - each with its own controls.
Extend the cache stanza to allow for this:
cache {
positive limit [ttl]
negative limit [ttl]
}
is now possible. This also add a cache_test.go in the toplevel test/
directory that exercises the caching path.
Fixes#260
e54c232c8c