Don't clone positions when checking is batch completed

[saitiart]

try_into_completed() cloned self.positions on every call so it could hand self back in case of Err. Since maybe_finished() runs after each chunk, this is rather inneficient since it deep clones all responses and then just discards them.

I replaced it with a check (is_complete) and after a consuming conversion (into_completed) only if the check passes.
maybe_finished now uses get_mut and removes the entry once instead of remove + re-insert on every call.
On the performance side, I havent had a way to measure the speedup and its presumably not significant, but it reduces redundant memory operations.