bridge: Don't pay attention to columns that vswitchd doesn't need.
Not replicating unneeded columns has some value in avoiding CPU time and
bandwidth to the database. In ovs-vswitchd, setting cur_cfg as write-only
also have great value in avoiding extra reconfiguration steps. When
ovs-vsctl is used in its default mode this essentially avoids half of the
reconfigurations that ovs-vswitchd currently does. What happens now is:
1. ovs-vsctl updates the database and increments next_cfg.
2. ovs-vswitchd notices the change to the database, reconfigures
itself, then increments cur_cfg to match next_cfg.
3. The database sends the change to cur_cfg back to ovs-vswitchd.
4. ovs-vswitchd reconfigures itself a second time.
By not replicating cur_cfg we avoid step 3 and save a whole reconfiguration
step.
Also, now that the database contains interface statistics, this avoids
reconfiguring every time that statistics are updated.