Handle concurrent finishing of the same build

There is a slight possibility that the queue monitor and a builder
thread simultaneously decide to mark a build as finished. That's fine,
as long as we ensure the DB update is idempotent (as ensured by doing
"update Builds set finished = 1 ... where finished = 0").

src/hydra-queue-runner/sync.hh

@@ -28,6 +28,9 @@ private:
 
 public:
 
+    Sync() { }
+    Sync(const T & data) : data(data) { }
+
     class Lock
     {
     private: