Port fsp engine to new tractor stream api

piker/fsp/__init__.py

@@ -167,7 +167,8 @@ async def cascade(
             # Increment the underlying shared memory buffer on every
             # "increment" msg received from the underlying data feed.
 
-            async for msg in await feed.index_stream():
+            async with feed.index_stream() as stream:
+                async for msg in stream:
 
                     new_len = len(src.array)