Memory leak with gorutiones in new relic plugin

[LinMAD]

When there is no internet connection, go routines will be not closed in the plugin for the new relic.

That affects memory overflow and crashes system.

Review the following code usages from here:

https://github.com/LinMAD/BitAccretion/blob/master/extension/newrelic/provider.go
`

go func(g *model.Graph) {
	log.Debug(fmt.Sprintf("Harvesting data from NewRelic API..."))
	nr.fetchMetricsWithGraph(g, log)
	isProcessed <- true
}(g)

for {
	select {
	case <-isProcessed:
		nr.pluginHealth = model.HealthNormal
		return *g, nil
	case <-timeout:
		log.Error("Timeout got from NewRelic provider...")
		nr.pluginHealth = model.HealthWarning
		return *g, nil
	}
}

`