Fixed issue with subscriptions not being cleared in external scan

If GDA moved outer axis and scans were fast (300Hz), CPU usage would
mount during the scan. This was because on each run() a new subscription
was added to underlying completedSteps attributes. Fix this by removing
the existing subscriptions before adding a new one

malcolm/parts/builtin/runnablechildpart.py

@@ -58,6 +58,7 @@ def run(self, task, update_completed_steps):
             update_completed_steps (func): The function we should call when
                 completedSteps should be updated
         """
+        task.unsubscribe_all()
         task.subscribe(
             self.child["completedSteps"], update_completed_steps, self)
         match_future = self._wait_for_postrun(task)