10% drop of replicateTo=1 KV throughput on 6.5.0-1816

Test: 
YCSB Workload A (50/50 R/W), 3 nodes, 12 vCPU, replicateTo=1

Results:
6.5.0-1815: 50K ops/sec
6.5.0-1816: 45K ops/sec

Report:
**http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=hebe_650-1816_access_8171&snapshot=hebe_650-1815_access_dc1e

Logs:
6.5.0-1815: http://perf.jenkins.couchbase.com/job/hebe/2604/console
6.5.0-1816: http://perf.jenkins.couchbase.com/job/hebe/2603/console

changelog for 1816:
http://172.23.120.24/builds/latestbuilds/couchbase-server/mad-hatter/1816/CHANGELOG

Seems like only KV ReplicateTo=1 case is affected
http://showfast.sc.couchbase.com/#/timeline/Linux/kv/ycsb/all