Multi-node XDCR test doesn't report replication_changes_left correctly on build 7.1.0-1885

The XDCR test will monitor replication_changes_left after XDCR starts. In this run, xdcr reported replication_changes_left = 0, and it resulted in wrong performance number reported. 

Avg. initial XDCR rate (items/sec), 5 -> 5 (2 source nozzles, 4 target nozzles), 1 bucket x 1G x 1KB, DGM, s=1, c=1000

build: 7.1.0-1885

Job: http://perf.jenkins.couchbase.com/job/titan/12877/

2021-12-13T23:33:11 [INFO] Adding a remote cluster: 172.23.96.100

2021-12-13T23:33:11 [INFO] Starting replication with parameters {'replicationType': 'continuous', 'fromBucket': 'bucket-1', 'toBucket': 'bucket-1', 'toCluster': 'perf'}

2021-12-13T23:33:11 [INFO] Getting remote clusters

2021-12-13T23:33:11 [INFO] Monitoring XDCR queues: bucket-1

2021-12-13T23:33:33 [INFO] replication_changes_left reached 0

In a good run, replication_changes_left will decrease gradually. 

http://perf.jenkins.couchbase.com/job/titan/12397/

2021-11-01T17:41:31 [INFO] Adding a remote cluster: 172.23.96.100

2021-11-01T17:41:31 [INFO] Starting replication with parameters {'replicationType': 'continuous', 'fromBucket': 'bucket-1', 'toBucket': 'bucket-1', 'toCluster': 'perf'}

2021-11-01T17:41:31 [INFO] Getting remote clusters

2021-11-01T17:41:31 [INFO] Monitoring XDCR queues: bucket-1

2021-11-01T17:41:53 [INFO] replication_changes_left = 991,768,354

2021-11-01T17:41:55 [INFO] replication_changes_left = 990,671,216

2021-11-01T17:41:58 [INFO] replication_changes_left = 988,192,619