make ColumnVector public

ColumnVector is an internal interface in Spark SQL, which is only used for vectorized parquet reader to represent the in-memory columnar format.

In Spark 2.3 we want to make ColumnVector public, so that we can provide a more efficient way for data exchanges between Spark and external systems. For example, we can use ColumnVector to build the columnar read API in data source framework, we can use ColumnVector to build a more efficient UDF API, etc.

We also want to introduce a new ColumnVector implementation based on Apache Arrow(basically just a wrapper over Arrow), so that external systems(like Python Pandas DataFrame) can build ColumnVector very easily.