Eviction server not help evict pages sometimes

In WT-2453 we found that for nearly all workloads, having the eviction server help evict pages resulted in dramatic improvements. In one case, where the number of threads (both worker and app) is less than the number of cores, this results in a degradation (compared to develop before that branch). In that one case, having the server immediately go to refill the queue restores performance. We need to come up with some way to calculate if we're in that situation and have the server thread skip helping.