tools/rados: use the monotonic clock in rados bench

tools/rados: use the monotonic clock in rados bench

Use the steady (CLOCK_MONOTONIC) clock in the bencher. The clock is used
to measure the passage of time between events, making the monotonic clock
more suited. The system clock is affected by all changes to the system's wall
clock time, which makes it unfit to compare events times relatively to each
other.

Fixes http://tracker.ceph.com/issues/21375

Signed-off-by: Mohamad Gebai <mgebai@suse.com>