Map-Reduce: Mapper sometimes receives null value
ISPN-4471 MapReduceTask: memory leak with useIntermediateSharedCache = true
ISPN-4453 MapReduceTask#executeAsynchronously() isn't asynchronous
ISPN-1951 Use MERGE3 in the default UDP configuration

TEST_PING needed some changes, as MERGE3 doesn't trigger Discovery.findMembers()
Smaller timer pool and queue.
Still causes lots of failures.