Part of https://emqx.atlassian.net/browse/EMQX-10942
The goal is to help make it clear to the caller of `next` what to do next: if the iterator
should still be used or if no new messages will ever come out of it.
From:
```erlang
-spec next(iterator()) -> {value, binary(), iterator()} | none | {error, closed}.
```
To:
```erlang
-spec next(iterator()) -> {ok, iterator(), [binary()]} | end_of_stream.
-spec next(iterator(), pos_integer()) -> {ok, iterator(), [binary()]} | end_of_stream.
```
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