Enable backpressure during data-feed layer startup to avoid overruns

piker/data/feed.py

@@ -1467,7 +1467,7 @@ async def maybe_open_feed(
             'tick_throttle': kwargs.get('tick_throttle'),
 
             # XXX: super critical to have bool defaults here XD
-            'backpressure': kwargs.get('backpressure', True),
+            # 'backpressure': kwargs.get('backpressure', True),
             'start_stream': kwargs.get('start_stream', True),
         },
         key=fqsn,

tests/test_services.py

@@ -176,5 +176,5 @@ def test_ensure_ems_in_paper_actors(
     ) as exc_info:
         trio.run(main)
 
-    cancel_msg: str = '_emsd_main was remotely cancelled by its caller'
+    cancel_msg: str = '_emsd_main()` was remotely cancelled by its caller'
     assert cancel_msg in exc_info.value.args[0]