ISPN-4690 Upgrade to Scala 2.11.2

* Minor compilation adjutsments were required, including: switching
to new scala.concurrent classes added in 2.10, extend Serializable
trait instead of annotation, and use refactored Java conversion
methods.
* Scala XML module has been separated, so REST server needs a new
dependency, which it has also been added to Infinispan server as
a new module.
ISPN-3645 StateTransferLargeObjectTest hangs randomly

Add a more focused and reliable test: RemoteGetStressTest.