80th percentile 2i "stale = true" scan latency increases from 2ms to 160ms when there is background ops in KV.

Steps to reproduce:
1)Run Scan latency test,https://github.com/couchbase/perfrunner/blob/master/tests/secondary_scanlatency20M.test, without background KV workload.
capture the scan latency number.
2)Run Scan latency test with default test config. capture scan latency number. Run http://ci.sc.couchbase.com/job/secondary/197/console
3)Latency increases from 2ms to 160ms when there is background KV workload.