bridge: Don't pay attention to columns that vswitchd doesn't need.

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.