ovs-xapi-sync: Rerun processing when a db update arrives during a commit.

ovs-xapi-sync: Rerun processing when a db update arrives during a commit.

The logic in ovs-xapi-sync didn't handle the case where ovsdb-server sends
a database update before it replies to a transaction that ovs-xapi-sync
sent, like this:

ovs-xapi-sync              ovsdb-server
-------------              ------------

                      .
                      .
                      .
transaction request  --->
                     <---  database contents update
                     <---  transaction reply
                      .
                      .
                      .

The update was not lost but ovs-xapi-sync would not process it until the
database changed again.

Bug #10082.
Reported-by: Krishna Miriyala <krishna@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>