Improve performance of eviction-heavy workloads by dynamically controlling the number of eviction threads

The number of eviction server threads significantly affects performance of eviction heavy workloads. The number of threads yielding the best performance is not trivial to set, and varies depending on the workload, the hardware and the OS. The idea is to control the number of eviction server threads dynamically, using the amount of work done and (possibly) the degree of lock contention as heuristics.