http listener refactor

in this commit:

- chunks out the http request body to avoid making very large
  allocations.
- establishes a limit for the maximum http request body size that the
  listener will accept.
- utilizes a pool of byte buffers to reduce GC pressure.

internal/buffer/buffer.go

@@ -1,6 +1,8 @@
 package buffer
 
 import (
+	"sync"
+
 	"github.com/influxdata/telegraf"
 )
 
@@ -11,6 +13,8 @@ type Buffer struct {
 	drops int
 	// total metrics added
 	total int
+
+	sync.Mutex
 }
 
 // NewBuffer returns a Buffer
@@ -61,11 +65,13 @@ func (b *Buffer) Add(metrics ...telegraf.Metric) {
 // the batch will be of maximum length batchSize. It can be less than batchSize,
 // if the length of Buffer is less than batchSize.
 func (b *Buffer) Batch(batchSize int) []telegraf.Metric {
+	b.Lock()
 	n := min(len(b.buf), batchSize)
 	out := make([]telegraf.Metric, n)
 	for i := 0; i < n; i++ {
 		out[i] = <-b.buf
 	}
+	b.Unlock()
 	return out
 }