wire: Add large tx deserialize benchmark.

This adds a benchmark for deserializing a large transaction that is
often referred to as the megatransaction since it is the largest Bitcoin
transaction mined to date.  It consists of 5569 inputs and 1 output and
its hash is:

bb41a757f405890fb0f5856228e23b715702d714d59bf2b1feb70d8b2b4e3e08.

This is being done so there is a benchmark that tests more of a
worst-case scenario which is a better candidate for identifying and
testing improvements.

The following benchmark results shows the how much more intensive this
transaction is over the existing mock transaction:

DeserializeTx       1000000    1751 ns/op      376 B/op     16 allocs/op
DeserializeTxLarge  300     5093980 ns/op  1672829 B/op  33430 allocs/op

wire/bench_test.go

@@ -6,7 +6,9 @@ package wire
 
 import (
 	"bytes"
+	"compress/bzip2"
 	"io/ioutil"
+	"os"
 	"testing"
 	"time"
 )
@@ -306,7 +308,7 @@ func BenchmarkWriteTxIn(b *testing.B) {
 }
 
 // BenchmarkDeserializeTx performs a benchmark on how long it takes to
-// deserialize a transaction.
+// deserialize a small transaction.
 func BenchmarkDeserializeTx(b *testing.B) {
 	buf := []byte{
 		0x01, 0x00, 0x00, 0x00, // Version
@@ -338,7 +340,27 @@ func BenchmarkDeserializeTx(b *testing.B) {
 	var tx MsgTx
 	for i := 0; i < b.N; i++ {
 		tx.Deserialize(bytes.NewReader(buf))
+	}
+}
+
+// BenchmarkDeserializeTxLarge performs a benchmark on how long it takes to
+// deserialize a very large transaction.
+func BenchmarkDeserializeTxLarge(b *testing.B) {
+	// tx bb41a757f405890fb0f5856228e23b715702d714d59bf2b1feb70d8b2b4e3e08
+	// from the main block chain.
+	fi, err := os.Open("testdata/megatx.bin.bz2")
+	if err != nil {
+		b.Fatalf("Failed to read transaction data: %v", err)
+	}
+	defer fi.Close()
+	buf, err := ioutil.ReadAll(bzip2.NewReader(fi))
+	if err != nil {
+		b.Fatalf("Failed to read transaction data: %v", err)
+	}
 
+	var tx MsgTx
+	for i := 0; i < b.N; i++ {
+		tx.Deserialize(bytes.NewReader(buf))
 	}
 }