20% increased latency of BF11 no-index

BF11 query:

SELECT DISTINCT 1 FROM (SELECT * FROM ChirpMessages cm ORDER BY send_time ) AS foo;

Detected by weekly tests, reproduced both on 2 nodes and 4 nodes setup.

logs:

https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-oceanus-1318/172.23.96.205.zip
https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-oceanus-1318/172.23.96.57.zip
https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-oceanus-1318/172.23.96.5.zip
https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-oceanus-1318/172.23.96.7.zip
https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-oceanus-1318/172.23.96.8.zip
https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-oceanus-1318/172.23.96.9.zip

Comparison to 6.0.0:

http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=oceanus_601-2014_access_a04f&snapshot=oceanus_600-1693_access_b406