Merge pull request alteryx#140 from aarondav/merge-75

Manually merge BlockManager refactor alteryx#75

PR alteryx#75 was unfortunately not cherry-picked into 0.8.0, which caused a build breakage earlier and now again due to later changes being cherry-picked. This merges alteryx#75 and replays alteryx#139 on top of it. I have confirmed that alteryx#75 has made exactly the changes it did on master. So long as all BlockManager patches from master should have gone into branch-0.8, this PR should do the right thing.

Author: rxin