Duplicate iterator buffers (#823)

The slice that pebble.Iterator returns for Key() and Value() requests
are owned by the iterator. So it can be reused or garbage collected anytime.

We should make sure that we own those slices by copying the data first

db/pebble/iterator.go

@@ -19,12 +19,24 @@ func (i *iterator) Valid() bool {
 
 // Key : see db.Transaction.Iterator.Key
 func (i *iterator) Key() []byte {
-	return i.iter.Key()
+	key := i.iter.Key()
+	if key == nil {
+		return nil
+	}
+	buf := make([]byte, len(key))
+	copy(buf, key)
+	return buf
 }
 
 // Value : see db.Transaction.Iterator.Value
 func (i *iterator) Value() ([]byte, error) {
-	return i.iter.ValueAndErr()
+	val, err := i.iter.ValueAndErr()
+	if err != nil || val == nil {
+		return nil, err
+	}
+	buf := make([]byte, len(val))
+	copy(buf, val)
+	return buf, nil
 }
 
 // Next : see db.Transaction.Iterator.Next