ovn-controller: Persist desired conntrack groups.

ovn-controller: Persist desired conntrack groups.

With incremental processing of logical flows desired conntrack groups
are not being persisted.  This patch adds this capability, with the
side effect of adding a ds_clone method that this capability leverages.

Signed-off-by: Ryan Moats <rmoats@us.ibm.com>
Reported-by: Guru Shetty <guru@ovn.org>
Reported-at: http://openvswitch.org/pipermail/dev/2016-July/076320.html
Fixes: 70c7cfe ("ovn-controller: Add incremental processing to lflow_run and physical_run")
Acked-by: Flavio Fernandes <flavio@flaviof.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>