verion for source from buffer
Merge pull request #25 from zillabyte/xiao/version

@jakequist, @sashikanth, @nkarnik -- allow sourcing from different versions of a relation
fix test
Merge pull request #26 from zillabyte/jake/master

@qiongyu -- fix tests
fix test -- don't let targetparllelism > maxparallelism
revert
Merge pull request #27 from zillabyte/jake/master

@qiongyu, @nkarnik -- fix test round II