[SPARK-9517][SQL] BytesToBytesMap should encode data the same way as UnsafeExternalSorter

BytesToBytesMap current encodes key/value data in the following format:
```
8B key length, key data, 8B value length, value data
```

UnsafeExternalSorter, on the other hand, encodes data this way:
```
4B record length, data
```

As a result, we cannot pass records encoded by BytesToBytesMap directly into UnsafeExternalSorter for sorting. However, if we rearrange data slightly, we can then pass the key/value records directly into UnsafeExternalSorter:
```
4B key+value length, 4B key length, key data, value data
```

Author: Reynold Xin <rxin@databricks.com>

Closes #7845 from rxin/kvsort-rebase and squashes the following commits:

5716b59 [Reynold Xin] Fixed test.
2e62ccb [Reynold Xin] Updated BytesToBytesMap's data encoding to put the key first.
a51b641 [Reynold Xin] Added a KV sorter interface.