network: preallocate buffer for message

```
name            old time/op    new time/op    delta
MessageBytes-8     740ns ± 0%     684ns ± 2%   -7.58%  (p=0.000 n=10+10)

name            old alloc/op   new alloc/op   delta
MessageBytes-8    1.39kB ± 0%    1.20kB ± 0%  -13.79%  (p=0.000 n=10+10)

name            old allocs/op  new allocs/op  delta
MessageBytes-8      11.0 ± 0%      10.0 ± 0%   -9.09%  (p=0.000 n=10+10)
```

Signed-off-by: Evgeniy Stratonikov <evgeniy@nspcc.ru>

pkg/network/message.go

@@ -182,6 +182,11 @@ func (m *Message) Encode(br *io.BinWriter) error {
 	if err := m.tryCompressPayload(); err != nil {
 		return err
 	}
+	growSize := 2 + 1 // header + empty payload
+	if m.compressedPayload != nil {
+		growSize += 8 + len(m.compressedPayload) // varint + byte-slice
+	}
+	br.Grow(growSize)
 	br.WriteB(byte(m.Flags))
 	br.WriteB(byte(m.Command))
 	if m.compressedPayload != nil {

pkg/network/message_test.go

@@ -52,6 +52,34 @@ func TestEncodeDecodeVersion(t *testing.T) {
 	require.NotEqual(t, len(expected.compressedPayload), len(uncompressed))
 }
 
+func BenchmarkMessageBytes(b *testing.B) {
+	// shouldn't try to compress headers payload
+	ep := &payload.Extensible{
+		Category:        "consensus",
+		ValidBlockStart: rand.Uint32(),
+		ValidBlockEnd:   rand.Uint32(),
+		Sender:          util.Uint160{},
+		Data:            make([]byte, 300),
+		Witness: transaction.Witness{
+			InvocationScript:   make([]byte, 33),
+			VerificationScript: make([]byte, 40),
+		},
+	}
+	random.Fill(ep.Data)
+	random.Fill(ep.Witness.InvocationScript)
+	random.Fill(ep.Witness.VerificationScript)
+	msg := NewMessage(CMDExtensible, ep)
+
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_, err := msg.Bytes()
+		if err != nil {
+			b.FailNow()
+		}
+	}
+}
+
 func TestEncodeDecodeHeaders(t *testing.T) {
 	// shouldn't try to compress headers payload
 	headers := &payload.Headers{Hdrs: make([]*block.Header, CompressionMinSize)}