Higher latency of range scan query with MOI

Recent  "pool value" code change https://github.com/couchbase/query/commit/5b09eb86d528209371a27734fa7e63483f054d30 caused increased latency of a range scan query. Also throughput drop as the result.

Reproduced only with MOI and covering index. Tests like  CI3 or  YCSB Workload E are good representation:

Avg. Query Throughput (queries/sec), CI3, Range Scan, Covering, MOI:
http://showfast.sc.couchbase.com/#/timeline/Linux/n1ql/Q1_Q3/MOI
**

Avg. Query Throughput (queries/sec), Q13, Workload E, MOI, not_bounded:
http://showfast.sc.couchbase.com/#/timeline/Linux/n1ql/pagination/MOI

For CI3:
 comparison between 6.5.0-1807 and  6.5.0-1808:
http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=iris_650-1807_access_5e3f&label=6.5.0-1807&snapshot=iris_650-1808_access_3290&label=6.5.0-1808

 CPU profiles:
6.5.0-1807: [http://perf.jenkins.couchbase.com/job/iris/12929/artifact/
]6.5.0-1808: http://perf.jenkins.couchbase.com/job/iris/12928/artifact/

 logs from 1808: 
https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-iris-12928/172.23.100.55.zip