GEOMESA-135 Convert ehCache to Guava cache

* Removed ehCache from the DeDuplicatingIterator, as well as references to ehCache in the pom files, and replaced with Guava cache.
Guava LoadingCaches do: http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/cache/LoadingCache.html#get(K)