More streamlined interface with Pillar.

Using a semaphore to ensure requests to Pillar aren't too parallel,
so that we can cancel requests faster.

blender_cloud/async_loop.py

@@ -4,6 +4,7 @@ import asyncio
 import traceback
 import concurrent.futures
 import logging
+import gc
 
 import bpy
 
@@ -53,6 +54,9 @@ def kick_async_loop(*args) -> bool:
                   len(all_tasks))
         stop_after_this_kick = True
 
+        # Clean up circular references between tasks.
+        gc.collect()
+
         for task_idx, task in enumerate(all_tasks):
             if not task.done():
                 continue
@@ -68,6 +72,9 @@ def kick_async_loop(*args) -> bool:
                 print('{}: resulted in exception'.format(task))
                 traceback.print_exc()
 
+            # for ref in gc.get_referrers(task):
+            #     log.debug('      - referred by %s', ref)
+
     loop.stop()
     loop.run_forever()