ovsdb: Fix one off error in tracking monitor changes

ovsdb: Fix one off error in tracking monitor changes

dbmon's changes should be stored with the next transaction number,
rather than the current transaction number.  This bug causes the
changes of a transaction stored in a monitor to be unnoticed by
the jsonrpc connections that is responsible for flush the monitor
content.

However, the bug was not noticed until it was exposed by a later
optimization patch: "avoid unnecessary call to ovsdb_monitor_get_update()."
The lack of optimization means that the update is still generated
when 'unflushed' equals to n_transactions + 1, which should have
indicated the monitor has been flushed already.

Signed-off-by: Andy Zhou <azhou@ovn.org>
Acked-by: Liran Schour <lirans@il.ibm.com>
Acked-by: Ben Pfaff <blp@ovn.org>