Default thread count of 1 is confusing

Currently the Katana (`libgalois`) thread pool is by default 1 thread. This is weird. The default thread pool size should probably be the detected number of physical cores: `katana::GetThreadPool().getMaxUsableThreads()`. This will require a little lazy initialization, but shouldn't be too hard.

This issue is important as it could make Katana look bad since Katana is by default sequential.