Implement telegraf's own full metric type

main reasons behind this:
- make adding/removing tags cheap
- make adding/removing fields cheap
- make parsing cheaper
- make parse -> decorate -> write out bytes metric flow much faster

Refactor serializer to use byte buffer

plugins/serializers/influx/influx.go

@@ -7,6 +7,6 @@ import (
 type InfluxSerializer struct {
 }
 
-func (s *InfluxSerializer) Serialize(metric telegraf.Metric) ([]string, error) {
-	return []string{metric.String()}, nil
+func (s *InfluxSerializer) Serialize(metric telegraf.Metric) ([]byte, error) {
+	return metric.Serialize(), nil
 }