Copy metrics for each configured output

This is for better thread-safety when running with multiple outputs,
which can cause very odd panics at very high loads

primarily this is to address #1432

closes #1432

plugins/serializers/graphite/graphite.go

@@ -20,7 +20,7 @@ type GraphiteSerializer struct {
 	Template string
 }
 
-func (s GraphiteSerializer) Serialize(metric telegraf.Metric) ([]string, error) {
+func (s *GraphiteSerializer) Serialize(metric telegraf.Metric) ([]string, error) {
 	out := []string{}
 
 	// Convert UnixNano to Unix timestamps