blockchain: Add benchmark for using a map vs a slice

Benchmark added to compare the performance of a map vs a slice when
fetching utxos.  The benchmark shows roughly 25% performance improvement
when using slices instead of a map.

blockchain/bench_test.go

@@ -8,6 +8,7 @@ import (
 	"testing"
 
 	"github.com/btcsuite/btcd/btcutil"
+	"github.com/btcsuite/btcd/wire"
 )
 
 // BenchmarkIsCoinBase performs a simple benchmark against the IsCoinBase
@@ -29,3 +30,33 @@ func BenchmarkIsCoinBaseTx(b *testing.B) {
 		IsCoinBaseTx(tx)
 	}
 }
+
+func BenchmarkUtxoFetchMap(b *testing.B) {
+	block := Block100000
+	transactions := block.Transactions
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		needed := make(map[wire.OutPoint]struct{}, len(transactions))
+		for _, tx := range transactions[1:] {
+			for _, txIn := range tx.TxIn {
+				needed[txIn.PreviousOutPoint] = struct{}{}
+			}
+		}
+	}
+}
+
+func BenchmarkUtxoFetchSlices(b *testing.B) {
+	block := Block100000
+	transactions := block.Transactions
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		needed := make([]wire.OutPoint, 0, len(transactions))
+		for _, tx := range transactions[1:] {
+			for _, txIn := range tx.TxIn {
+				needed = append(needed, txIn.PreviousOutPoint)
+			}
+		}
+	}
+}