fix: level slice are not ordered

[KKould]

ref: https://github.com/tonbo-io/tonbo/issues/152

[github-actions[bot]]

Read:

+--------------------------------------------+----------+---------+
|                                            | tonbo    | rocksdb |
+=================================================================+
| random range reads                         | 125472ms | 36790ms |
|--------------------------------------------+----------+---------|
| random range reads                         | 123029ms | 36812ms |
|--------------------------------------------+----------+---------|
| random range projection reads              | 51284ms  | 37400ms |
|--------------------------------------------+----------+---------|
| random range projection reads              | 52147ms  | 37320ms |
|--------------------------------------------+----------+---------|
| random range projection reads (4 threads)  | 39848ms  | 37397ms |
|--------------------------------------------+----------+---------|
| random range projection reads (8 threads)  | 38383ms  | 37390ms |
|--------------------------------------------+----------+---------|
| random range projection reads (16 threads) | 36052ms  | 35825ms |
|--------------------------------------------+----------+---------|
| random range projection reads (32 threads) | 33175ms  | 35870ms |
+--------------------------------------------+----------+---------+

[github-actions[bot]]

Write:

+-------------------------------+--------+---------+
|                               | tonbo  | rocksdb |
+==================================================+
| bulk load                     | 6325ms | 9676ms  |
|-------------------------------+--------+---------|
| individual writes             | 8292ms | 10657ms |
|-------------------------------+--------+---------|
| individual writes (4 threads) | 8233ms | 10988ms |
|-------------------------------+--------+---------|
| individual writes (8 threads) | 7855ms | 10794ms |
|-------------------------------+--------+---------|
| batch writes                  | 8157ms | 7214ms  |
|-------------------------------+--------+---------|
| batch writes (4 threads)      | 7888ms | 7261ms  |
|-------------------------------+--------+---------|
| batch writes (8 threads)      | 7823ms | 7231ms  |
|-------------------------------+--------+---------|
| removals                      | 3148ms | 3908ms  |
+-------------------------------+--------+---------+