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

[rxin]

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

[SparkQA]

Test build #39315 has finished for PR 7845 at commit 2d4ad05.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #39325 has finished for PR 7845 at commit 5716b59.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[rxin]

Going to merge this since @JoshRosen and I looked at this together.