Fix potential issues with negative changes left stats

There are three issues in xdcr that could lead to negative changes left stats:
1. retrieval of high seqno stats may fail for some vbuckets. In this case high seqno is set to 0. an error should be returned in this case.
2. when we encounter an error when computing changes left, we set changes left to -1. This does not reflect well on UI. A better way may be to continue to use the old changes left value.
3. We periodically re-compute and update runtime stats for paused replications.
(1). when source topology does not change, we recompute changes_left stats and update the corresponding value in stats overview.
(2). when source topology changes, we recompute docs_processed and changes_left stats. We only update the changes_left stats in stats overview, though. This could lead to incorrect docs_processed and incorrect, potentially negative changes_left stats. We should also update the docs_processed stats in this case.