ISPN-4453 MapReduceTask#executeAsynchronously() isn't asynchronous

Master only. Should be given a priority. See https://issues.jboss.org/browse/ISPN-4453 for more details
Got a point there but these MapReduceTask(s) are heavyweight object and they are usually thrown away after execution so these execution path gets executed essentially once. Had it been   some heavily used loop I would be all for it. TBH I actually copied some of these these lines from CacheImpl#putAsync