Major Description
When spark reads a data file into a WritableColumnVector, the memory allocated by the WritableColumnVectors is not freed until the VectorizedColumnReader completes.
It will save memory allocation time by reusing the allocated array objects. But it also takes up too many unused memory after the current large vector batch has been read.
Add a memory reserve policy for this scenario which will reuse the allocated array object for small column vectors and free the memory for huge column vectors.
