Disable auto-tuning of WiredTiger eviction threads

In WT-2898, we added code to automatically tune the number of eviction worker threads in WiredTiger. That work (a) assumed eviction.threads_max was reasonably large (e.g., 8), and (b) was driven by test workloads where the behavior at startup matches the long-term workload behavior.

To avoid MongoDB users being adversely affected by that change, we should disable it in 3.2 and 3.4 by changing the default eviction.threads_min=4 – i.e., min threads = max threads = 4.