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)
It's even worse than that: `Ehcache` class is in module `core` while `OnHeapStore` is in module `impl` and we do not want dependencies from `core` to `impl`