XDCR - changes_left stats could become negative during rebalancing

Note - this only applies to the Neo branch due to MB-45983 improvement

Before the MB-45983 improvement, statsMgr itself would have memcached clients to grab the Vb -> high seqno map. This means that once a pipeline is created, its list of VBs to calculate total_changes and docs_processed are static.

However, with MB-45983, statsMgr is now a subscriber to bucket topology service and as VBs are moved out during rebalancing, it is now possible for total_changes to be < than docs_processed (since docs_processed is static per pipeline).

The fix here is to take an intercept such that only the VBs that this node owns will be considered for calculating docs_processed as well as total_changes, and so that the stats will remain either 0 or positive.