ovs-ofctl, ovs-controller: Disable flow idle timeout by default.

ovs-ofctl, ovs-controller: Disable flow idle timeout by default.

Until now, flows set up by ovs-ofctl and by "ovs-controller --with-flows"
by default expired after 60 seconds of inactivity.  This was surprising,
especially in the latter case where one is normally trying to set up
permanent flows.  Even in the former case, however, we can't think of a
good reason that flows added by ovs-ofctl should expire by default.  So
this commit make flows permanent by default.

Reported-by: Michael Mao <mmao@nicira.com>