Add rate limiter for S3 API layer (#9196)

- total number of S3 API calls per server
- maximum wait duration for any S3 API call

This implementation is primarily meant for situations
where HDDs are not capable enough to handle the incoming
workload and there is no way to throttle the client.

This feature allows MinIO server to throttle itself
such that we do not overwhelm the HDDs.

cmd/endpoint.go

@@ -220,14 +220,28 @@ func (l EndpointZones) HTTPS() bool {
 	return l[0].Endpoints.HTTPS()
 }
 
-// Nodes - returns all nodes count
-func (l EndpointZones) Nodes() (count int) {
+// NEndpoints - returns all nodes count
+func (l EndpointZones) NEndpoints() (count int) {
 	for _, ep := range l {
 		count += len(ep.Endpoints)
 	}
 	return count
 }
 
+// Hosts - returns list of unique hosts
+func (l EndpointZones) Hosts() []string {
+	foundSet := set.NewStringSet()
+	for _, ep := range l {
+		for _, endpoint := range ep.Endpoints {
+			if foundSet.Contains(endpoint.Host) {
+				continue
+			}
+			foundSet.Add(endpoint.Host)
+		}
+	}
+	return foundSet.ToSlice()
+}
+
 // Endpoints - list of same type of endpoint.
 type Endpoints []Endpoint