lib/db: Keep metadata better in sync (ref #6335) (#6337)

This adds metadata updates to the same write batch as the underlying
file change. The odds of a metadata update going missing is greatly
reduced.

Bonus change: actually commit the transaction in recalcMeta.

lib/db/lowlevel.go

@@ -124,11 +124,17 @@ func (db *Lowlevel) updateRemoteFiles(folder, device []byte, fs []protocol.FileI
 			return err
 		}
 
-		if err := t.Checkpoint(); err != nil {
+		if err := t.Checkpoint(func() error {
+			return meta.toDB(t, folder)
+		}); err != nil {
 			return err
 		}
 	}
 
+	if err := meta.toDB(t, folder); err != nil {
+		return err
+	}
+
 	return t.Commit()
 }
 
@@ -227,11 +233,17 @@ func (db *Lowlevel) updateLocalFiles(folder []byte, fs []protocol.FileInfo, meta
 			}
 		}
 
-		if err := t.Checkpoint(); err != nil {
+		if err := t.Checkpoint(func() error {
+			return meta.toDB(t, folder)
+		}); err != nil {
 			return err
 		}
 	}
 
+	if err := meta.toDB(t, folder); err != nil {
+		return err
+	}
+
 	return t.Commit()
 }