Don't allow interleaving with any other task when making a sync-lowered call

Author: lukewagner

design/mvp/canonical-abi/definitions.py

@@ -1395,9 +1395,7 @@ def return_thunk(results):
       nonlocal flat_results
       flat_results = lower_sync_values(subtask, MAX_FLAT_RESULTS, results, ft.result_types(), flat_args)
 
-    inst.thread.release()
     await callee(task, start_thunk, return_thunk)
-    await inst.thread.acquire()
 
     subtask.finish()
   else: