[Witness] Minor optimization to witnessing request (#927)

I noticed a deep lint warning about the string appending, and then
decided to modernize it and benchmark it. While I was there, I did it
properly to pre-allocate a byte array for the b64 encoding. It's 5x
faster and and uses a lot fewer memory allocations now.

```
// New
BenchmarkBuildRequestBody-24    	  533068	      2524 ns/op	    2082 B/op	       7 allocs/op

// Old
BenchmarkBuildRequestBody-24    	   95288	     12502 ns/op	   14301 B/op	      65 allocs/op
```

Author: mhutchinson

internal/witness/witness.go

@@ -291,23 +291,11 @@ func (w *witness) update(ctx context.Context, cp []byte, size uint64, fetchProof
 			}
 		}
 
-		// The request body MUST be a sequence of
-		// - a previous size line,
-		// - zero or more consistency proof lines,
-		// - and an empty line,
-		// - followed by a [checkpoint][].
-		body := fmt.Sprintf("old %d\n", w.size)
-		for _, p := range proof {
-			body += base64.StdEncoding.EncodeToString(p) + "\n"
-		}
-		body += "\n"
-		body += string(cp)
-
 		start := time.Now()
 		nameAttr := witnessNameKey.String(w.verifier.Name())
 		witnessReqsTotal.Add(ctx, 1, metric.WithAttributes(nameAttr))
 
-		req, err := http.NewRequestWithContext(ctx, http.MethodPost, w.url, strings.NewReader(body))
+		req, err := http.NewRequestWithContext(ctx, http.MethodPost, w.url, w.buildRequestBody(proof, cp))
 		if err != nil {
 			return nil, fmt.Errorf("failed to construct request to %q: %v", w.url, err)
 		}
@@ -390,3 +378,26 @@ func (w *witness) update(ctx context.Context, cp []byte, size uint64, fetchProof
 		}
 	})
 }
+
+// buildRequestBody formats the request body for the witness.
+// The request body MUST be a sequence of
+// - a previous size line,
+// - zero or more consistency proof lines,
+// - and an empty line,
+// - followed by a [checkpoint][].
+func (w *witness) buildRequestBody(proof [][]byte, cp []byte) io.Reader {
+	var b bytes.Buffer
+	fmt.Fprintf(&b, "old %d\n", w.size)
+
+	// Preallocate for 32 byte SHA256 nodes
+	dst := make([]byte, base64.StdEncoding.EncodedLen(32))
+
+	for _, p := range proof {
+		base64.StdEncoding.Encode(dst, p)
+		b.Write(dst)
+		b.WriteByte('\n')
+	}
+	b.WriteByte('\n')
+	b.Write(cp)
+	return &b
+}

internal/witness/witness_internal_test.go

@@ -0,0 +1,19 @@
+package witness
+
+import (
+	"testing"
+)
+
+func BenchmarkBuildRequestBody(b *testing.B) {
+	w := &witness{size: 0}
+	proof := make([][]byte, 20)
+	for i := range proof {
+		proof[i] = []byte("representative-proof-data-block")
+	}
+	cp := []byte("checkpoint-data-is-usually-longer-and-has-multiple-lines\n")
+
+	b.ResetTimer()
+	for b.Loop() {
+		_ = w.buildRequestBody(proof, cp)
+	}
+}