Improvements to benchmarks

1. Add CreateFakeBundleAdjustmentJacobian to create
   bundle adjustment structured jacobians.
2. sparse_linear_operator_benchmark -> spmv_benchmark
3. Refactor spmv_benchmark to use CreateFakeBundleAdjustmentJacobian.
4. Add BA and unstructured Jacobian variants of spmv benchmarks
5. Add BA and unstructured Jacobian benchmarks for the block Jacobi
   preconditioner.
6. Fix BlockSparseMatrix::ToCompressedRowSparseMatrix to add the
   row and column block structure to the output.

Change-Id: I737a3d7d82dad665b1f2e7886c2c3aedd702ffcd