Handle broken mem chan on `Actor._push_result()`

When backpressure is used and a feeder mem chan breaks during msg
delivery (usually because the IPC allocating task already terminated)
instead of raising we simply warn as we do for the non-backpressure
case.

Also, add a proper `Actor.is_arbiter` test inside `._invoke()` to avoid
doing an arbiter-registry lookup if the current actor **is** the
registrar.

tractor/_runtime.py

@@ -826,7 +826,12 @@ class Actor:
 
             if ctx._backpressure:
                 log.warning(text)
+                try:
                     await send_chan.send(msg)
+                except trio.BrokenResourceError:
+                    # XXX: local consumer has closed their side
+                    # so cancel the far end streaming task
+                    log.warning(f"{chan} is already closed")
             else:
                 try:
                     raise StreamOverrun(text) from None
@@ -1371,8 +1376,9 @@ async def async_main(
         actor.lifetime_stack.close()
 
         # Unregister actor from the arbiter
-        if registered_with_arbiter and (
-                actor._arb_addr is not None
+        if (
+            registered_with_arbiter
+            and not actor.is_arbiter
         ):
             failed = False
             with trio.move_on_after(0.5) as cs: