1. Fixed race in MRTask2/Frame:
Frame.vecs() had a race in allowing threads to change the member _vecs variable before filling it, so one thread could fill the _vecs array and continue with its work, while sec on thread would start filling in the _vecs array and the first thread would get partially updated _vecs array including nulls. Fixed the vecs() call to update the _vecs only after completely filled plus added vecs() call in the MRTask2 to happen before setupLocal(0 is invoke so that the _vecs are filled only once for the MRTask instead of several racing updates from map calls.

2. Updated Jobs:
 cancel call is now blocking.
 moved onCancelled() call to be invoked in onSuccess method of the Atomic, to avoid potential multiple execution.
Merge branch 'master' of github.com:0xdata/h2o
Updated Job.cancel to handle case when job is missing.
Fixed ParseDataset to remove job when done.
Merge remote-tracking branch 'origin/master' into vers