ofproto: Disable timeouts for flows added by ofproto_add_flow().

ofproto: Disable timeouts for flows added by ofproto_add_flow().

None of the existing callers of ofproto_add_flow() want an idle timeout,
but ofproto_add_flow() was giving them a 5-second timeout anyway.  I don't
know how this worked properly--in-band will patiently add the flow back
every few seconds anyway, but the bridge doesn't do that.

Also add an explanatory comment to ofproto_add_flow().