ISPN-4372 Map/Reduce performance is dependent on cache value size
ISPN-4440 Remove setMaxCollectorSize API from MapReduceTask
ISPN-3981 Enhance grouping API to support group manipulation methods

* First implementation. Added getGroup() and removeGroup() methods
ISPN-4452 DummyInMemoryStore should respect fetch arguments on process method.
ISPN-3938 AdvancedAsyncCacheLoader.process() concurrency issues

Simplify ExecutorAllCompletionService to track the tasks itself instead of
using an ExecutorCompletionService.