[B.P. 7.1.5] - changes_left does not go to 0 in collection migration with OSO mode

This can be reproduced by running 5a_multi_target_migration_tc.shlib. If we turn off OSO, changes_left will go to 0.

In ThroughSeqnoTracker, there are multiple DualSortedSeqnoListWithLock used that assume seqnos are sorted. In OSO mode, that's no longer the case. That causes list search to fail to find results. Here is one example:

https://github.com/couchbase/goxdcr/blob/cc2da8364000df680ca476fb765ba334425f8ada/service_impl/through_seqno_tracker_service.go#L1707

where searching seqno 7 in the list [7, 2] will fail to find its index.