Make ovs-vswitchd report when it is done configuring; make ovs-vsctl wait.

Make ovs-vswitchd report when it is done configuring; make ovs-vsctl wait.

Until now the ovsdb-based vswitch has provided no way to know when it has
finished applying the configuration from the database.  This commit
introduces a way:

  * The client who wants to wait increments the "next_cfg" column of the
    Open_vSwitch record.

  * When ovs-vswitchd finishes reconfiguring, it sets the value of the
    "cur_cfg" column to that of the "next_cfg" column.

  * The client waits until the "cur_cfg" column is at least as great as
    the value it set into "next_cfg".

This allows us to drop the 5-second sleep in interface-reconfigure.