index: add garbage collection benchmark

Allocates an index and repeatedly triggers the GC.

internal/index/master_index_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"math/rand"
+	"runtime"
 	"testing"
 	"time"
 
@@ -323,6 +324,17 @@ func BenchmarkMasterIndexEach(b *testing.B) {
 	}
 }
 
+func BenchmarkMasterIndexGC(b *testing.B) {
+	mIdx, _ := createRandomMasterIndex(b, rand.New(rand.NewSource(0)), 100, 10000)
+
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		runtime.GC()
+	}
+	runtime.KeepAlive(mIdx)
+}
+
 var (
 	snapshotTime = time.Unix(1470492820, 207401672)
 	depth        = 3