Return sortKey from query result, to allow resuming query.
Return sortKey from query result, to allow resuming query.

This is required to be able to retrieve more than 500 changes using the query interface.

markon/pygerrit2,morucci/pygerrit,sonyxperiadev/pygerrit,dpursehouse/pygerrit,benjiii/pygerrit,gferon/pygerrit2,dpursehouse/pygerrit2