Improve WiredTiger eviction performance

In WT-2764, WiredTiger's eviction logic changed to notice cases where only clean or only dirty pages need to be evicted to meet targets. However, some of the heuristics about how many pages to evict still assume that we should select the number of pages to put on queues based on the total size in cache of each tree. This leads to performance regressions when only searching for dirty pages.