ISPN-5013 Stored Script Execution 

https://issues.jboss.org/browse/ISPN-5013

WARNING: this is merely a preview of stored script execution. Please contribute to the design doc:
https://github.com/infinispan/infinispan/wiki/Script-Execution-Design
and obviously comment on the code here.
We need to call des.shutdown() in finally clause. This is an implementation of ExecutorService afterall and shutdown will release all the resources used by ExecutorService.