ofproto-dpif: keep slow path flow time stamp up-to-date

ofproto-dpif: keep slow path flow time stamp up-to-date

Noting updating slow path subfacet's time stamp can cause their datapath
flows deleted periodically. For example, CFM datapath flow have usespace
actions that are handled in dpif slow path. They are deleted and
recreated periodically without the fix.

This bug are not obvious during normal operation. Deleted CFM flow
would cause CFM packets to be handled by flow miss handler which will
reinstall the flow in the datapath. The only potentially observable
behavior is that when the user space is overwhelmed with flow miss packets,
the periodic CFM miss packets may get stuck behind other miss packets,
cause tunnel flapping.

Ben refactored the patch to its current form.

Reported-by: Guolin Yang <gyang@nicira.com>
Co-authored-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Andy Zhou <azhou@nicira.com>