Increase the low watermark default.

The difference between the high watermark and low watermark is pretty high (10%) and the cost of pulling items back off disk is costly. In a few production environments where there is slow growth, when the cluster hits the high watermark it has a massive impact on the workload. I suspect it would be better if the low watermark is much closer to the high. Scanning memory a few more time will always be faster than pulling data from disk.