Use multiple threads for renaming

A single thread is much too slow for hundreds, or more, rename tasks.

With at least two, one thread can be loading while another is being processed. The times when nothing is happening are reduced significantly.

tubesync/tubesync/settings.py

@@ -209,6 +209,9 @@ if MAX_RUN_TIME < 600:
 
 DOWNLOAD_MEDIA_DELAY = 60 + (MAX_RUN_TIME / 20)
 
+if RENAME_SOURCES or RENAME_ALL_SOURCES:
+    BACKGROUND_TASK_ASYNC_THREADS += 1
+
 if BACKGROUND_TASK_ASYNC_THREADS > MAX_BACKGROUND_TASK_ASYNC_THREADS:
     BACKGROUND_TASK_ASYNC_THREADS = MAX_BACKGROUND_TASK_ASYNC_THREADS