ovsdb-client: Serialize columns in predictable order on "monitor" command.

ovsdb-client: Serialize columns in predictable order on "monitor" command.

The "monitor" command goes to some trouble to write its output in a
predictable order, so that test programs can reliably compare it against
expectations.  This commit fixes up one part that was missing, which is
that the columns were not being ordered predictably (it depended on
hash order, which differs between big-endian and little-endian systems).
It also updates the test suite to expect the new order.