[Minor] Fix style, formatting and naming in BlockManager etc.

This is a precursor to a bigger change. I wanted to separate out the relatively insignificant changes so the ultimate PR is not inflated.

(Warning: this PR is full of unimportant nitpicks)
This is the only non-trivial part about this PR. I basically factored out the matching of `data` to the appropriate value type, so we don't need to do this for each case (i.e. `useMemory`, `useOffHeap`, `useDisk`)