_scheduler/jobs/cleanupjob.py: Update cache size while processing

Updates the known cache size in the main process while the cleanup
process is ongoing, so that the status indicators update live
while the cleanup happens.

buildstream/_scheduler/jobs/cleanupjob.py

@@ -28,7 +28,18 @@ class CleanupJob(Job):
         self._artifacts = context.artifactcache
 
     def child_process(self):
-        return self._artifacts.clean()
+        def progress():
+            self.send_message('update-cache-size',
+                              self._artifacts.get_cache_size())
+        return self._artifacts.clean(progress)
+
+    def handle_message(self, message_type, message):
+
+        # Update the cache size in the main process as we go,
+        # this provides better feedback in the UI.
+        if message_type == 'update-cache-size':
+            self._artifacts.set_cache_size(message)
+            return True
 
     def parent_complete(self, status, result):
         if status == JobStatus.OK: