[HOS-333] Send a "reload" message to the frontend after state expiry

1. a state instance expires on the backing store
2. frontend attempts to process an event against the expired token and gets a
   fresh instance of the state without router_data set
3. backend sends a "reload" message on the websocket containing the event and
   immediately stops processing
4. in response to the "reload" message, frontend sends
   [hydrate, update client storage, on_load, <previous_event>]

This allows the frontend and backend to re-syncronize on the state of the app
before continuing to process regular events.

If the event in (2) is a special hydrate event, then it is processed normally
by the middleware and the "reload" logic is skipped since this indicates an
initial load or a browser refresh.

reflex/.templates/web/utils/state.js

@@ -454,6 +454,10 @@ export const connect = async (
       queueEvents(update.events, socket);
     }
   });
+  socket.current.on("reload", async (event) => {
+    event_processing = false;
+    queueEvents([...initialEvents(), JSON5.parse(event)], socket);
+  })
 
   document.addEventListener("visibilitychange", checkVisibility);
 };

reflex/app.py

@@ -73,6 +73,7 @@ from reflex.event import (
     EventSpec,
     EventType,
     IndividualEventType,
+    get_hydrate_event,
     window_alert,
 )
 from reflex.model import Model, get_db_status
@@ -1259,6 +1260,21 @@ async def process(
         )
         # Get the state for the session exclusively.
         async with app.state_manager.modify_state(event.substate_token) as state:
+            # When this is a brand new instance of the state, signal the
+            # frontend to reload before processing it.
+            if (
+                not state.router_data
+                and event.name != get_hydrate_event(state)
+                and app.event_namespace is not None
+            ):
+                await asyncio.create_task(
+                    app.event_namespace.emit(
+                        "reload",
+                        data=format.json_dumps(event),
+                        to=sid,
+                    )
+                )
+                return
             # re-assign only when the value is different
             if state.router_data != router_data:
                 # assignment will recurse into substates and force recalculation of