Don't error on sub removal attempts, feeds need backpressure

piker/data/feed.py

@@ -356,7 +356,10 @@ async def open_feed_bus(
                 f'Stopping {symbol}.{brokername} feed for {ctx.chan.uid}')
             if tick_throttle:
                 n.cancel_scope.cancel()
+            try:
                 bus._subscribers[symbol].remove(sub)
+            except ValueError:
+                log.warning(f'{sub} for {symbol} was already removed?')
 
 
 @asynccontextmanager
@@ -520,7 +523,12 @@ async def open_feed(
 
         ) as (ctx, (init_msg, first_quotes)),
 
-        ctx.open_stream() as stream,
+        ctx.open_stream(
+            # XXX: be explicit about stream backpressure since we should 
+            # **never** overrun on feeds being too fast, which will
+            # pretty much always happen with HFT XD
+            backpressure=True
+        ) as stream,
 
     ):
         # we can only read from shm