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.
A quick unit test might be in order.  Granted, it's a deficiency that there wasn't already a unit test in place, but it should be a small enough addition that we needn't create a new ticket for it.  Thoughts?