ofproto: Improve resolution of flow stats and removed messages (OpenFlow 1.0)

ofproto: Improve resolution of flow stats and removed messages (OpenFlow 1.0)

OpenFlow 1.0 increases the resolution of flow stats and flow removed messages
from seconds to (potentially) nanoseconds.  The spec stats that only
millisecond granularity is required, so that's all we provide at this
time.  Increasing to nanoseconds would require more significant code
change and would not provide an appreciable improvement in real world
use.

NOTE: OVS at this point is not wire-compatible with OpenFlow 1.0 until
the final commit in this OpenFlow 1.0 set.