Move fields under mutex

tsdb/engine/tsm1/engine.go

@@ -40,16 +40,15 @@ const (
 type Engine struct {
 	mu                sync.RWMutex
 	done              chan struct{}
+	index             *tsdb.DatabaseIndex // TODO(benbjohnson): needs to be moved entirely into engine.
+	measurementFields map[string]*tsdb.MeasurementFields
+
 	wg sync.WaitGroup
 
 	path      string
 	logger    *log.Logger
 	logOutput io.Writer
 
-	// TODO(benbjohnson): Index needs to be moved entirely into engine.
-	index             *tsdb.DatabaseIndex
-	measurementFields map[string]*tsdb.MeasurementFields
-
 	WAL            *WAL
 	Cache          *Cache
 	Compactor      *Compactor