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/aggregators/minmax/minmax_test.go

@@ -5,10 +5,11 @@ import (
 	"time"
 
 	"github.com/influxdata/telegraf"
+	"github.com/influxdata/telegraf/metric"
 	"github.com/influxdata/telegraf/testutil"
 )
 
-var m1, _ = telegraf.NewMetric("m1",
+var m1, _ = metric.New("m1",
 	map[string]string{"foo": "bar"},
 	map[string]interface{}{
 		"a": int64(1),
@@ -24,7 +25,7 @@ var m1, _ = telegraf.NewMetric("m1",
 	},
 	time.Now(),
 )
-var m2, _ = telegraf.NewMetric("m1",
+var m2, _ = metric.New("m1",
 	map[string]string{"foo": "bar"},
 	map[string]interface{}{
 		"a":        int64(1),