[SPARK-11792] [SQL] [FOLLOW-UP] Change SizeEstimation to KnownSizeEstimation...

[SPARK-11792] [SQL] [FOLLOW-UP] Change SizeEstimation to KnownSizeEstimation and make estimatedSize return Long instead of Option[Long]

https://issues.apache.org/jira/browse/SPARK-11792

The main changes include:
* Renaming `SizeEstimation` to `KnownSizeEstimation`. Hopefully this new name has more information.
* Making `estimatedSize` return `Long` instead of `Option[Long]`.
* In `UnsaveHashedRelation`, `estimatedSize` will delegate the work to `SizeEstimator` if we have not created a `BytesToBytesMap`.

Since we will put `UnsaveHashedRelation` to `BlockManager`, it is generally good to let it provide a more accurate size estimation. Also, if we do not put `BytesToBytesMap` directly into `BlockerManager`, I feel it is not really necessary to make `BytesToBytesMap` extends `KnownSizeEstimation`.

Author: Yin Huai <yhuai@databricks.com>

Closes #9813 from yhuai/SPARK-11792-followup.