* The original SparseRowMatrix use two fields to store each rows,
  which let code very confusing. Try to extract a RowBuffer class,
  for SparseRowMatrix data storage, and manage auto-growth logic.

ISSUE=4586495

git-svn-id: https://svn.baidu.com/idl/trunk/paddle@1408 1ad973e4-5ce8-4261-8a94-b56d1f490c56