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).

Fixes #3065 
Fixes #3289 

backport follows
It was kept by mistake, I had to run the tests in a heavily parallel mode and this one was always timing out, just forgot to put it back to quick.