cbbackupmgr: Restore performance degradation of ~40% between 6.5.0-4000 and 6.5.0-4064

I'm seeing a degradation of cbbackupmgr restore throughput of around 40% across both ForestDB and SQLite.

From an initial analysis, looks like this performance degradation is cbbackumpgr side as resource utilisation and write queues Couchbase Server side are lower than in previous runs, so it seems that throughput driven by cbbackupmgr is lower.

Ops/s shows a significant drop between 6.5.0-4000 and 6.5.0-4064:

Run: http://perf.jenkins.couchbase.com/job/leto/10552/
cbmonitor report: http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=leto_650-4064_restore_0071

Attached are client and server side logs for an SQLite run.