Map-Reduce: Mapper sometimes receives null value
ISPN-4471 MapReduceTask: memory leak with useIntermediateSharedCache = true
ISPN-4453 MapReduceTask#executeAsynchronously() isn't asynchronous
ISPN-3938 AdvancedAsyncCacheLoader.process() concurrency issues

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