LSM primary changes impact parallel-pop-lsm load time

The changes for WT-3012 have caused a 500% increase in the load time in the parallel-pop-lsm.wtperf test. The start of the times is this build:

http://build.wiredtiger.com:8080/job/wiredtiger-perf-lsm/1568/

The plots are http://build.wiredtiger.com:8080/job/wiredtiger-perf-lsm/plot/

Time went from a very consistent, 17.25 seconds to a very consistent 112.77 seconds.