50% drop of Ephemeral bucket ejection rate (nru eviction)

The ejection rate dropped from 120K to 88K after following change (build 6.0.0-1650):

kv_engine changed from 90eadc1556511a7f35118d45890070bd53df67b1 to alice [+] 69082c1 MB-31175: Ephemeral HTTombstonePurger underflows age check
http://review.couchbase.org/#/c/99744/

Test:
http://perf.jenkins.couchbase.com/job/ares/8670/console

Logs:

https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-ares-8670/172.23.133.11.zip
https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-ares-8670/172.23.133.12.zip
https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-ares-8670/172.23.133.13.zip
https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-ares-8670/172.23.133.14.zip