caching in encoders

[floybix]

Some encoders run non-trivial computations: currently the coordinate encoder and unique (random) encoder. We should cache the results for performance.

For JVM there is the very nice core.cache with a choice of expiry strategies: https://github.com/clojure/core.cache/

There doesn't seem to be a good clojurescript port, but it would be easy to pull out LRUCache from https://github.com/clojure/core.cache/blob/master/src/main/clojure/clojure/core/cache.clj