Allow searchRate trigger to delete replicas

Currently SearchRateTrigger generates events when search rate thresholds are exceeded, and ComputePlanAction computes ADDREPLICA actions in response - adding replicas should allow the search rate to be reduced across the increased number of replicas.

However, once the peak load period is over the collection is left with too many replicas, which unnecessarily tie cluster resources. SearchRateTrigger should detect situations like this and generate events that should cause some of these replicas to be deleted.

SearchRateTrigger should use hysteresis to avoid thrashing when the rate is close to the threshold.