Add gtod_cpu option for pinning gettimeofday() to a single CPU

Add gtod_cpu option for pinning gettimeofday() to a single CPU

Similar to what real life products sometimes do, offload gettimeofday()
calls to a single CPU and have that update the current time into a shared
memory location. This option pins a specific CPU for that job, and excludes
it from participating in any of the IO jobs.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>