worker: Wait until datasets are written before quitting

Avoids a race condition in worker_impl.py where HDF5 dataset saving was
cut off before it finished for large datasets.

artiq/master/worker_impl.py

@@ -307,9 +307,9 @@ def main():
             elif action == "analyze":
                 try:
                     exp_inst.analyze()
-                    put_completed()
                 finally:
                     write_results()
+                put_completed()
             elif action == "examine":
                 examine(ExamineDeviceMgr, ExamineDatasetMgr, obj["file"])
                 put_completed()