Collectors in parallel (#74)

* Provide info to CollectorManager whether the collector can be executed in parallel with others

* Split serial and parallel collectors. Read in parallel first

collectors/nfsMetric.go

@@ -114,6 +114,7 @@ func (m *nfsCollector) MainInit(config json.RawMessage) error {
 	m.data = make(map[string]NfsCollectorData)
 	m.initStats()
 	m.init = true
+	m.parallel = true
 	return nil
 }