Expiration reaper is not enabled until lifespan or max-idle is set

If a cache is added where entities are with and without expiration by external processes it is expected that the reaper purge the data from memory and datastore.

As the reaper is only enabled for the cache if the element
<expiration interval="millis" lifespan="millis" max-idle="millis"/>
has one of the attributes lifespan or max-idle set to a value >0
The expired entities will not be removed from the memory or datastore.

This is an unexpected behaviour and might end in a memory bottleneck if a server is up for a long time.