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

@@ -350,12 +350,13 @@ def main():
             elif action == "analyze":
                 try:
                     exp_inst.analyze()
-                    put_completed()
                 finally:
                     # browser's analyze shouldn't write results,
                     # since it doesn't run the experiment and cannot have rid
                     if rid is not None:
                         write_results()
+
+                put_completed()
             elif action == "examine":
                 examine(ExamineDeviceMgr, ExamineDatasetMgr, obj["file"])
                 put_completed()