REF: rename _data-->_mgr
At the sprint there was discussion of trimming down the exposed surface of internals.  Among the things that makes this difficult is identifying exactly what that surface is.  This makes it feasiable to grep for all accesses by using the unambiguous `._mgr` instead of the overloaded `._data`.
> what is the upside of changing this?

Discussed in the OP: it is hard to tell right now where BlockManager is accessed across the code-base (most of which we want to remove).  With this, you can find it immediately by grepping. > > what is the upside of changing this?
> 
> Discussed in the OP: it is hard to tell right now where BlockManager is accessed across the code-base (most of which we want to remove). With this, you can find it immediately by grepping.

I am not sure this is a positive return given the churn on other packages.