feat: Throttle meta request for large chat size (#10518)

For large accounts with huge volumes of messages, it can be very
wasteful to make the meta request so often. It also puts un-necessary
load on the DB bombarding it with so many requests. This PR fixes it by
throttling the requests to 5 seconds for all users with more than 1000
accessible chats.

### Why not cache this value in the backend?

Well, it's a bit tricky, since a user can have different permissions
over inboxes and can see different chats, maintaining a cache for each
of them is not effective, besides the requests will reach the server
anyway.

app/javascript/dashboard/store/modules/specs/conversationStats/mutations.spec.js

@@ -14,6 +14,7 @@ describe('#mutations', () => {
         mineCount: 1,
         unAssignedCount: 1,
         allCount: 2,
+        updatedOn: expect.any(Date),
       });
     });
   });