Changes TBB to use tbb::task_arena instead of tbb::task_scheduler_init.

Fixes the current implementation where the desired number of threads may
not be honored if another tbb::task_scheduler_init is instantiated. We
are using tbb::task_arena to solve this which is only available in newer
versions of TBB.

Also increases the performance by not creating/destroying the TBB setup
via tbb::task_scheduler_init on every iteration evaluation. This
increases the performance in single threaded mode using TBB by 10x.

By not specifically calling tbb::task_scheduler_init, this will either
respect any active tbb::task_scheduler_init instantiations or use the
default TBB settings which is hardware dependent. Ceres will honor the
user's requested number of threads through the task_arenas.

Tested via compiling with TBB enabled and ran the unit tests.

Change-Id: I5538407563449cdb5a0eaf8b8ccab62263912110