[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)
Your change moves this class to the `storage` package (instead of being an inner class of `MemoryStore`). Is there a reason for this change? If it is going to be inside of `storage` it might make sense to call it `MemoryEntry` because otherwise we can't ever name another class `org.apache.spark.storage.Entry`.