Issue #147 - Add non-bulk Ehcache unit tests

Adds unit tests for the following Ehcache methods:

* get
* put
* containsKey
* remove(key)
* clear
* iterator
* putIfAbsent
* remove(key, value)
* replace(key, value)
* replace(key, oldValue, newValue)
Chris Dennis: are you suggesting that this should be called ERROR instead? [comment](https://github.com/cljohnso/ehcache3/commit/9096c8936a58ed373ad978fc6aeb0225b7b34422#commitcomment-8798486)

Clifford Johnson: Just want to make sure that recording a FAILURE observation is correct -- in this case, if the resilience strategy was successful, the CacheWriter recorded the correct value and the local store is hopefully in a state that a subsequent call will return the recorded value. From the Ehcache's caller's perspective, the replace() call "worked". [comment](https://github.com/cljohnso/ehcache3/commit/9096c8936a58ed373ad978fc6aeb0225b7b34422#commitcomment-8803788)