XDCR doesn't report replication_changes_left correctly on build 7.1.0-1831

Test: Avg. initial XDCR rate (items/sec), 1 -> 2 changes to 2 -> 2 (2 source nozzles, 4 target nozzles), 1 bucket x 200M x 1KB

Job: http://perf.jenkins.couchbase.com/job/titan/12833/ 

Build: 7.1.0-1831

Logs:

https://s3.amazonaws.com/bugdb/jira/qe/collectinfo-2021-12-10T201638-ns_1%40172.23.96.105.zip

https://s3.amazonaws.com/bugdb/jira/qe/collectinfo-2021-12-10T201638-ns_1%40172.23.96.106.zip

https://s3.amazonaws.com/bugdb/jira/qe/collectinfo-2021-12-10T201732-ns_1%40172.23.96.100.zip

https://s3.amazonaws.com/bugdb/jira/qe/collectinfo-2021-12-10T201732-ns_1%40172.23.96.101.zip

In this test, we will monitor number of replication_changes_left to make sure replication is finished (replication_changes_left=0). In the runs with build 7.1.0-1831, we see the number of replication_changes_left becomes negative so the runs will never finish. The issue is reproducible.

2021-12-10T11:27:33 [INFO] replication_changes_left = 12,859,568

2021-12-10T11:27:35 [INFO] replication_changes_left = 11,318,963

2021-12-10T11:27:38 [INFO] replication_changes_left = -2,798,374

2021-12-10T11:27:40 [INFO] replication_changes_left = -2,798,374

2021-12-10T11:27:42 [INFO] replication_changes_left = -2,798,374

2021-12-10T11:27:44 [INFO] replication_changes_left = -2,798,374

2021-12-10T11:27:46 [INFO] replication_changes_left = -6,127,946

2021-12-10T11:27:48 [INFO] replication_changes_left = -6,127,946

2021-12-10T11:27:50 [INFO] replication_changes_left = -6,127,946

We get replication_changes_left from the xdcr stats.

http://172.23.96.105:8091/pools/default/buckets/@xdcr-bucket-1/stats 

"replication_changes_left":[-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000,-90625000],"