Do benchmarks with read-only Tx after committing write sometimes

For the Rows benchmark, we were continuing to use the original writable
transaction, meaning RBF was spending all its time looking up dirty
pages in the transaction's dirty page cache rather than working with
the disk in any way. It wasn't clear whether this was hurting or
helping performance, but it was clear that it wasn't testing the
"real" workload use case, where queries are done against the RBF
file rather than the dirty page cache.

Modify the benchmark to test it both ways for comparison. Answer:
The RBF file is faster than the in-memory map (!).

fragment_internal_test.go

@@ -3276,8 +3276,8 @@ func TestGetZipfRowsSliceRoaring(t *testing.T) {
 	f.Clean(t)
 }
 
-func prepareSampleRowData(b *testing.B, bits int, rows uint64, width uint64) (*fragment, Tx) {
-	f, _, tx := mustOpenFragment(b, "i", "f", viewStandard, 0, "none")
+func prepareSampleRowData(b *testing.B, bits int, rows uint64, width uint64) (*fragment, *Index, Tx) {
+	f, idx, tx := mustOpenFragment(b, "i", "f", viewStandard, 0, "none")
 	for i := 0; i < bits; i++ {
 		data := getUniformRowsSliceRoaring(rows, int64(rows)+int64(i), 0, width)
 		err := f.importRoaringT(tx, data, false)
@@ -3285,12 +3285,13 @@ func prepareSampleRowData(b *testing.B, bits int, rows uint64, width uint64) (*f
 			b.Fatalf("creating sample data: %v", err)
 		}
 	}
-	return f, tx
+	return f, idx, tx
 }
 
 type txFrag struct {
 	rows, width uint64
 	tx          Tx
+	idx         *Index
 	frag        *fragment
 }
 
@@ -3305,7 +3306,9 @@ func benchmarkRowsOnTestcase(b *testing.B, ctx context.Context, txf txFrag) {
 	}
 }
 
-func BenchmarkRows(b *testing.B) {
+// benchmarkRowsMaybeWritable uses a local flag which shadows the package-scope
+// const writable. Neat, huh.
+func benchmarkRowsMaybeWritable(b *testing.B, writable bool) {
 	var depths = []uint64{384, 2048}
 	testCases := make([]txFrag, 0, len(depths))
 	defer func() {
@@ -3315,11 +3318,19 @@ func BenchmarkRows(b *testing.B) {
 	}()
 	bg := context.Background()
 	for _, rows := range depths {
-		frag, tx := prepareSampleRowData(b, 3, rows, ShardWidth)
+		frag, idx, tx := prepareSampleRowData(b, 3, rows, ShardWidth)
+		if !writable {
+			err := tx.Commit()
+			if err != nil {
+				b.Fatalf("error committing sample data: %v", err)
+			}
+			tx = idx.holder.txf.NewTx(Txo{Write: writable, Index: idx, Fragment: frag, Shard: 0})
+		}
 		testCases = append(testCases, txFrag{
 			rows:  rows,
 			width: ShardWidth,
 			frag:  frag,
+			idx:   idx,
 			tx:    tx,
 		})
 	}
@@ -3329,6 +3340,14 @@ func BenchmarkRows(b *testing.B) {
 		})
 	}
 }
+func BenchmarkRows(b *testing.B) {
+	b.Run("writable", func(b *testing.B) {
+		benchmarkRowsMaybeWritable(b, true)
+	})
+	b.Run("readonly", func(b *testing.B) {
+		benchmarkRowsMaybeWritable(b, false)
+	})
+}
 
 // getZipfRowsSliceRoaring generates a random fragment with the given number of
 // rows, and 1 bit set in each column. The row each bit is set in is chosen via