Avoid passing evicted blocks throughout BlockManager / CacheManager

This is a minor refactoring task.

Currently when we attempt to put a block in, we get back an array buffer of blocks that are dropped in the process. We do this to propagate these blocks back to our TaskContext, which will add them to its TaskMetrics so we can see them in the SparkUI storage tab properly.

Now that we have TaskContext.get, we can just use that to propagate this information. This simplifies a lot of the signatures and gets rid of weird return types like the following everywhere:

ArrayBuffer[(BlockId, BlockStatus)]