ovsdb-server: Send update for _version for changes due to weak ref removal.

ovsdb-server: Send update for _version for changes due to weak ref removal.

When a row was deleted, and that caused the transaction manager to remove
a remaining weak reference to the row, and no other change had been made
to the row in that transaction, and a client was monitoring modifications
of the _version column in the row, then the monitor update for the column
did not include the old contents of the _version column, even though it
should have.  This commit fixes the problem.

Probably most clients only look at the new value of the column, if they
monitor _version at all, and this bug is really old, so it's probably
not a serious bug.

Found by inspection.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>