ovs-vswitchd: An option to wait for userspace flow restore to complete.

ovs-vswitchd: An option to wait for userspace flow restore to complete.

While upgrading openvswitch, it helps to restore openflow flows before
starting packet processing.  Typically we want to restart openvswitch,
add the openflow flows and then start packet processing.

To do this, we look for the other_config:flow-restore-wait column
in the Open_vSwitch table during startup. If set as true, we disable
receiving packets from the datapath, expiring or flushing flows and
running any periodic ofproto activities. This option does not prevent
the addition and deletion of ports. Once this option is set to false,
we return to normal processing.

An upcoming commit will use this feature in Open vSwitch startup scripts.

Bug #16086.
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>