Consider LSM trees when tracking dirty content in cache

We recently made a change in WiredTiger so that the default settings only allow up to 20% of the cache to become dirty, and drive that limit down prior to checkpoints.

That may not play nicely with LSM trees, since LSM trees rely on being able to pin content into cache. We should consider whether to exclude LSM trees from the dirty calculation, or at least have a set of suggested configuration settings for applications using LSM trees.