ofproto: Add support for barrier command (OpenFlow 0.9)

ofproto: Add support for barrier command (OpenFlow 0.9)

OpenFlow 0.9 introduces the concept of the barrier command.  When the
controller sends a Barrier Request, the switch is not allowed to respond
with a Barrier Reply until it has finished processing any other commands
that preceded it.  This commit provides that support.

NOTE: OVS at this point is not wire-compatible with OpenFlow 0.9 until the
final commit in this OpenFlow 0.9 set.