[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)
I looked this part over and it seemed good. Did you consider making these two things `val`'s and just assigning them with an if statement?

```
val (returnValues, blockStore) {
  if (level.useMemory) {
    (true, memoryStore)
  }
  ...
}
```