Change storage in BlockRandomAccessSparseMatrix

- TripletSparseMatrix in BlockRandomAccessSparseMatrix is replaced with
   BlockSparseMatrix
 - BlockSparseMatrix::ToCompressedRowSparseMatrix is performed in a
   direct sort-less way

Change-Id: Ib951fda1b9394050e2c47a9721172c5e3c674801

internal/ceres/schur_complement_solver.h

@@ -186,6 +186,7 @@ class CERES_NO_EXPORT SparseSchurComplementSolver final
   std::vector<Block> blocks_;
   std::unique_ptr<SparseCholesky> sparse_cholesky_;
   std::unique_ptr<BlockRandomAccessDiagonalMatrix> preconditioner_;
+  std::unique_ptr<CompressedRowSparseMatrix> crs_lhs_;
   Vector cg_solution_;
   Vector* scratch_[4] = {nullptr, nullptr, nullptr, nullptr};
 };