MapReduce bugfixes

Added enforcement to have a partition table before the actual job is started and found missing volatiles on the reducer implementations of the unittests (tests were mostly copied from castmapr where the volatile keyword was not needed).

This should fix: 
- https://github.com/hazelcast/hazelcast/issues/3065
- https://github.com/hazelcast/hazelcast/issues/3289

backport follows