Embedded JCache listener does not correctly return the previous value

See discussion here.

The cache entry modified embedded event does not easily expose the previous value. To get the previous value, you need to listen for isPre=true event and cache the value. This is not user friendly.

The cache entry modified embedded event should probably expose a method to retrieve previous value regardless of whether isPre is true or false.

The JCache TCK currently only verifies that the previous value is non null, so we can just the current value and the TCK passes, but this is not correct.