ReplicaSetMonitor should check replicas at most twice per second

Under some circumstances ReplicaSetMonitor can call isMaster on replica set members as fast as possible, instead of respecting the Server Discovery and Monitoring Spec's minHeartbeatFrequencyMS of 500ms. This risks overloading replicas and doesn't provide much benefit at detecting elections sooner. ReplicaSetMonitor should respect minHeartbeatFrequencyMS.