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/cpufreqCpuinfoMetric.go

@@ -48,6 +48,7 @@ func (m *CPUFreqCpuInfoCollector) Init(config json.RawMessage) error {
 	m.setup()
 
 	m.name = "CPUFreqCpuInfoCollector"
+	m.parallel = true
 	m.meta = map[string]string{
 		"source": m.name,
 		"group":  "CPU",