Preallocate memory for the message header in WriteMessageN and the

serialization buffer in MsgTx's TxSha().

Benchmarking shows this is slightly faster due to avoiding the extra
garbage collection in addition to less peak memory usage.

Before: BenchmarkTxShaOld  500000         5626 ns/op
After:  BenchmarkTxShaNew  500000         5457 ns/op

message.go

@@ -218,8 +218,8 @@ func WriteMessageN(w io.Writer, msg Message, pver uint32, btcnet BitcoinNet) (in
 	// Encode the header for the message.  This is done to a buffer
 	// rather than directly to the writer since writeElements doesn't
 	// return the number of bytes written.
-	var hw bytes.Buffer
-	writeElements(&hw, hdr.magic, command, hdr.length, hdr.checksum)
+	hw := bytes.NewBuffer(make([]byte, 0, MessageHeaderSize))
+	writeElements(hw, hdr.magic, command, hdr.length, hdr.checksum)
 
 	// Write header.
 	n, err := w.Write(hw.Bytes())

msgtx.go

@@ -157,9 +157,9 @@ func (msg *MsgTx) TxSha() (ShaHash, error) {
 	// cause a run-time panic.  Also, SetBytes can't fail here due to the
 	// fact DoubleSha256 always returns a []byte of the right size
 	// regardless of input.
-	var buf bytes.Buffer
+	buf := bytes.NewBuffer(make([]byte, 0, msg.SerializeSize()))
+	_ = msg.Serialize(buf)
 	var sha ShaHash
-	_ = msg.Serialize(&buf)
 	_ = sha.SetBytes(DoubleSha256(buf.Bytes()))
 
 	// Even though this function can't currently fail, it still returns