Multithread DENSE_SCHUR

Replace the global lock in BlockRandomAccessDenseMatrix
with a per cell lock.

Change-Id: Iddbe38616157b6e0d3770eede3335a056c3ba18c
diff --git a/internal/ceres/solver_impl_test.cc b/internal/ceres/solver_impl_test.cc
index 2471ea2..24860d2 100644
--- a/internal/ceres/solver_impl_test.cc
+++ b/internal/ceres/solver_impl_test.cc
@@ -561,7 +561,7 @@
       SolverImpl::CreateLinearSolver(&options, &error));
   EXPECT_TRUE(solver != NULL);
   EXPECT_EQ(options.linear_solver_type, DENSE_SCHUR);
-  EXPECT_EQ(options.num_linear_solver_threads, 1);
+  EXPECT_EQ(options.num_linear_solver_threads, 2);
 }
 
 TEST(SolverImpl, CreateIterativeLinearSolverForDogleg) {