Support no-disconnect on `open_aio_clients()` exit

Allows for easier restarts of certain `trio` side tasks without killing
the `asyncio`-side clients; support via flag.

Also fix a bug in `Client.bars()`: we need to return the duration on the
empty bars case..

piker/brokers/ib/api.py

@@ -412,7 +412,7 @@ class Client:
             # ``end_dt`` which exceeds the ``duration``,
             # - a timeout occurred in which case insync internals return
             # an empty list thing with bars.clear()...
-            return [], np.empty(0)
+            return [], np.empty(0), dt_duration
             # TODO: we could maybe raise ``NoData`` instead if we
             # rewrite the method in the first case? right now there's no
             # way to detect a timeout.
@@ -1086,6 +1086,7 @@ async def load_aio_clients(
     # retry a few times to get the client going..
     connect_retries: int = 3,
     connect_timeout: float = 0.5,
+    disconnect_on_exit: bool = True,
 
 ) -> dict[str, Client]:
     '''
@@ -1227,6 +1228,7 @@ async def load_aio_clients(
     finally:
         # TODO: for re-scans we'll want to not teardown clients which
         # are up and stable right?
+        if disconnect_on_exit:
             for acct, client in _accounts2clients.items():
                 log.info(f'Disconnecting {acct}@{client}')
                 client.ib.disconnect()