Magma YCSB has low loading ops rate at the beginning

Magma (160 workers): http://perf.jenkins.couchbase.com/job/magma-nvme-ycsb/246/
Magma (320 workers): http://perf.jenkins.couchbase.com/job/magma-nvme-ycsb/245/

In Magma YCSB load tests, we observed that the ops rate is low during the first 1000 seconds. Increasing number of workers can resolve this issue. However, increasing number of workers doesn't improve average throughput. Therefore, we shouldn't need so many workers. Moreover, ep_dcp_replica_items_remaining is higher when we increase workers.

http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=magma-nvme_700-3154_custom_load_4342&label=160worker&snapshot=magma-nvme_700-3154_custom_load_0471&label=320worker#7d953b61958286f86d6bacb30330e47f

http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=magma-nvme_700-3154_custom_load_4342&label=160worker&snapshot=magma-nvme_700-3154_custom_load_0471&label=320worker#2c9be7495d498bf4ae151733781c8069

Note that we observed this issue on build 7.0.0-2792 and build 7.0.0-3154. We didn't try an old build.