fix: return Vec<Datafile> instead of HashMap

[ZENOTME]

I find that task writer don't need to return HashMap<StructValue,DataFile>.

User will use it like following and it probably don't need to know the partition details.

let table = create_table();

let task_writer = table.task_writer();

// TaskWriter partition for user, they don't need to know the details.
task_writer.writer(records);

let res = task_writer.close();

let transaction = transaction::new(&mut table);

// Just pass the result to transaction.
transaction.append_datafile(res);

transaction.commit();