Hung wtperf run of parallel-pop-btree.wtperf.

Appears to be stuck in the populate phase with a cache size of 209715200 and 38869716 dirty leaf bytes (just under 20%). Was run against git rev d6c7a001dded7b000a08d8675592f912ccdbab79

Run:
http://build.wiredtiger.com:8080/job/wiredtiger-perf-btree/1415/