YCSB performance regressions in master vs. 3.4 and 3.5.

The throughput for Mongo Master-WT Mongo Master: small docs with 50u/50r - zipifan dropped 31% with the build #1267, Aug 10. On a positive note, this build shows a large improvement in the large docs throughput.

There is also some amount of regression with build #1234, Jun 6 (appx 2%) and build #1240, Jun 20 (appx 5%). These smaller ones are also seen in all of the other small docs throughput perf tests.