update_index: wait for all pool workers to finish

There was a race condition where update_index() would return
before all of the workers had finished updating Solr. This
manifested itself most frequently as Travis failures
for the multiprocessing test (see django-haystack#1001).

Author: acdha

haystack/management/commands/update_index.py

@@ -242,7 +242,8 @@ def update_backend(self, label, using):
             if self.workers > 0:
                 pool = multiprocessing.Pool(self.workers)
                 pool.map(worker, ghetto_queue)
-                pool.terminate()
+                pool.close()
+                pool.join()
 
             if self.remove:
                 if self.start_date or self.end_date or total <= 0: