Defines an externally persisted entry. External stores that keep the data in serialised form should return an
MarshalledEntry that contains the data in binary form (ByteBuffer) and unmarshall it lazily when
getKey/Value/Metadata are invoked. This approach avoids unnecessary object (de)serialization e.g
when the entries are fetched from the external store for the sole purpose of being sent over the wire to
another requestor node.