dpctl: Add new 'ct-bkts' command.

dpctl: Add new 'ct-bkts' command.

With the command:
 ovs-appctl dpctl/ct-bkts
shows the number of connections per bucket.

By using a threshold:
 ovs-appctl dpctl/ct-bkts gt=N
for each bucket shows the number of connections when they
are greater than N.

Signed-off-by: Antonio Fischetti <antonio.fischetti@intel.com>
Signed-off-by: Bhanuprakash Bodireddy <bhanuprakash.bodireddy@intel.com>
Co-authored-by: Bhanuprakash Bodireddy <bhanuprakash.bodireddy@intel.com>
Signed-off-by: Darrell Ball <dlu998@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>