vswitchd: Don't reset idle timer when updating flows.

vswitchd: Don't reset idle timer when updating flows.

When a flow was revalidated, we would use a OFPFC_ADD flow_mod message to
change the flow's actions.  However, this resets the idle-timer countdown.
In extreme circumstances, such as when VMs are being continuously migrated,
this meant that completely idle flows would never expire, because their
idle timers would keep getting reset more often than every 5 seconds, and
so the flow table would keep growing, never shrinking.

Now, when we revalidate an existing flow and update its actions, we use
an OFPFC_MODIFY_STRICT flow_mod message, which also updates actions but
does not reset the idle-timer countdown.