Focus eviction walks in largest trees

In SERVER-24580, there is evidence that eviction walks are still inefficient when there are many idle files in cache. The basic issue is that the list of handles has no information about the relative size in cache of trees, so eviction is just as likely to walk a tiny tree as a large tree.

WiredTiger has long had backoff for completely empty trees, so they are skipped on future eviction passes. We should consider a more nuanced approach so that very large trees get much more attention than very small trees.