Fix logic for discarding reconciliation if member rejoined

The current implementation of the new consumer discards the result of a reconciliation if the member rejoined, based on a comparison of the member epoch at the start and end of the reconciliation. If the epochs changed the reconciliation is discarded. This is not right because the member epoch could be incremented without an assignment change. This should be fixed to ensure that the reconciliation is discarded if the member rejoined, probably based on a flag that truly reflects that it went through a transition to joining while reconciling.