ISPN-4444 After state transfer, a node is able to read keys it no longer...

... owns from its data container

* mark the keys stored as L1
* this avoid read the keys that are going to be invalidated during the ST

https://issues.jboss.org/browse/ISPN-4444
Do we really need the flag on all the entries? Couldn't we implement `isL1Entry()` as `metadata instanceof L1Metadata`?