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This code is called in the hot path when Tornado is processing events.
As such, making this code performant is important. Profiling shows
that a significant portion of the time is spent calling asdict() to
serialize the UserMessageNotificationsData dataclass. In this case
`asdict` does several steps which we do not need, such as attempting
to recurse into its fields, and deepcopy'ing the values of the fields.
In our use case, these add a notable amount of overhead:
```py3
from zerver.tornado.event_queue import UserMessageNotificationsData
from dataclasses import asdict
from timeit import timeit
o = UserMessageNotificationsData(1, False, False, False, False, False, False, False, False, False, False, False)
%timeit asdict(o)
%timeit {**vars(o)}
```
Replace the `asdict` call with a direct access of the fields. We
perform a shallow copy because we do need to modify the resulting
fields.