Auto-tune checkpoint write-leaves phase

WiredTiger currently flushes dirty content from cache prior to starting a checkpoint. That was beneficial in some workloads, but appears detrimental (or at least wasted effort) in others. It would be good to add a mechanism that detects whether write-leaves is being useful and enables or disables it.

This concept could be extended to deciding whether to fsync handles prior to starting the checkpoint and other checkpoint optimizations as well.