ISPN-4692 Optimize externalizer for FileListCacheValue

https://issues.jboss.org/browse/ISPN-4692

Made FileListCacheValue Delta aware

DO NOT INTEGRATE YET: the WarmCacheTest is failing because of NPE when doing preload with DeltaAware values. I've sent a separate PR for that https://github.com/infinispan/infinispan/pull/2892