users may see incorrect "Outbound mutations" stat after topology change at source cluster (was: Rebalance in/out operation on Source cluster caused outbound replication mutations != 0 for long time while no write operation on source cluster)

[Test case]
./testrunner -i ./xdcr.1.ini -t xdcr.rebalanceXDCR.Rebalance.swap_rebalance_out_master,items=1000,rdirection=unidirection,ctopology=chain,doc-ops=update-delete,rebalance=source

[Test Exception]
======================================================================
FAIL: swap_rebalance_out_master (xdcr.rebalanceXDCR.Rebalance)
----------------------------------------------------------------------
Traceback (most recent call last):
File "pytests/xdcr/rebalanceXDCR.py", line 328, in swap_rebalance_out_master
elif self._replication_direction_str in "bidirection":
File "pytests/xdcr/xdcrbasetests.py", line 714, in verify_results
else:
File "pytests/xdcr/xdcrbasetests.py", line 683, in verify_xdcr_stats
timeout = max(120, end_time - time.time())
File "pytests/xdcr/xdcrbasetests.py", line 661, in __wait_for_mutation_to_replicate
AssertionError: Timeout occurs while waiting for mutations to be replicated

----------------------------------------------------------------------

[Test Steps]

1. Create 2-2 nodes Source and Destination clusters.
2. Create default bucket on both the clusters.
3. Setup CAPI mode XDCR from source-destination.
4. Load 1000 items on source cluster.
5. Do swap-rebalance master node on source cluster.
6. After rebalance is finished, wait for rebalance_changes_left to 0 on source side. --> Test failed here, getting rebalance_changes_left as 1 always on source cluster.
7. Verify items.

[Bug description]
Outbound replication mutations doesn't goes to 0 after rebalance.