Shuffle blocks for partial_fit wrappers

https://twitter.com/shoyer/status/912387291749822465

The incremental learn's are susceptible to bias when the blocks aren't IID (the dataset is ordered by time, say, and X or y is correlated with time).

How best to handle this? A full shuffle is costly, and unnecessary. A shuffle of the blocks (plus maybe an in-memory shuffle of each block) should be fine. Actually... I should check now if the scheduler goes through the blocks in order.