Use LRU cache as sliding window

The FIFO cache did not work when the threshold was increased from 1,000
to 99,000. We can't use a fifo-cache with a large threshold due to
StackOverFlow (http://stackoverflow.com/a/5294379/3366)
From their docs "increasing cache size does not benefit its efficiency"
(https://github.com/clojure/core.cache/blob/master/docs/FIFO.md)