ring: add benchmark for roundtrip

Running this on a RPi3 gives the following:

```
$ go test -run ^$ -bench . -benchmem -benchtime 30s
goos: linux
goarch: arm
BenchmarkRoundTrip-4   	    2000	  20178499 ns/op	   1.49 MB/s	     174 B/op	       3 allocs/op
PASS
```

ring/ring_test.go

@@ -205,3 +205,96 @@ func TestRoundTrip(t *testing.T) {
 		wg.Wait()
 	}
 }
+
+func BenchmarkRoundTrip(b *testing.B) {
+	const (
+		maxTimeouts = 100
+
+		len     = 50
+		size    = 150e3
+		timeout = 10 * time.Millisecond
+
+		frameLen = 30e3
+
+		writeDelay = 20 * time.Millisecond
+		readDelay  = 50 * time.Millisecond
+	)
+
+	// Allocated prior to timer reset since it is an
+	// amortised cost.
+	rb := NewBuffer(len, size, timeout)
+
+	// This is hoisted here to ensure the allocation
+	// is not counted since this is outside the control
+	// of the ring buffer.
+	buf := make([]byte, size+1)
+
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		var timeouts int
+	elements:
+		for {
+			err := rb.Next(timeout)
+			switch err {
+			case nil:
+				timeouts = 0
+			case ErrTimeout:
+				if timeouts > maxTimeouts {
+					b.Error("too many timeouts")
+					return
+				}
+				timeouts++
+			case io.EOF:
+				break elements
+			default:
+				b.Errorf("unexpected read error: %v", err)
+				return
+			}
+		reads:
+			for {
+				n, err := rb.Read(buf)
+				if n != 0 {
+					time.Sleep(readDelay) // Simulate slow data processing.
+				}
+				switch err {
+				case nil:
+				case io.EOF:
+					break reads
+				default:
+					b.Errorf("unexpected read error: %v", err)
+					return
+				}
+			}
+		}
+	}()
+
+	data := make([]byte, frameLen)
+
+	b.ResetTimer()
+	b.SetBytes(frameLen)
+
+	var dropped int
+	for i := 0; i < b.N; i++ {
+		time.Sleep(writeDelay) // Simulate slow data capture.
+		_, err := rb.Write(data)
+		switch err {
+		case nil:
+			dropped = 0
+		case ErrDropped:
+			if dropped > maxTimeouts {
+				b.Error("too many write drops")
+				return
+			}
+			dropped++
+		default:
+			b.Errorf("unexpected write error: %v", err)
+			return
+		}
+	}
+
+	rb.Close()
+
+	wg.Wait()
+}