Be slightly pickier about going into fail-open mode.

Be slightly pickier about going into fail-open mode.

Before, the amount of time disconnected was measured relative to the
last time we connected, that is, the last time connect(2) succeeded.
Thus, if we were connected for a long time, and then disconnected,
we would immediately go into fail-open mode.

This change make the disconnected time relative to the last OpenFlow
message received.  Thus, if we are connected for a long time and
receive plenty of packets, and then disconnect, there will still be
an opportunity to reconnect before failing open.