Avoid creating MarshallableEntry instances if no CacheWriter enabled

The (Dist)CacheWriterIntercepter is enabled if any loader/store is configured. The problem, is that the interceptor assumes that a CacheWriter is always configured. However, it is possible that only a CacheLoader has been configured, in which case when a write command visits the interceptor, MarshallableEntry instances are created unnecessarily in order to call the various PersistenceManager write operations.

To avoid unnecessary allocations and the marshalling of key/value/metadata that is never stored, we should ensure that calls to PersistenceManager write methods are only called if a CacheWriter is actually configured.