[SPARK-8968][SQL] external sort by the partition clomns when dynamic...

[SPARK-8968][SQL] external sort by the partition clomns when dynamic partitioning to optimize the memory overhead

Now the hash based writer dynamic partitioning show the bad performance for big data and cause many small files and high GC. This patch we do external sort first so that each time we only need open one writer.

before this patch:
![gc](https://cloud.githubusercontent.com/assets/7018048/9149788/edc48c6e-3dec-11e5-828c-9995b56e4d65.PNG)

after this patch:
![gc-optimize-externalsort](https://cloud.githubusercontent.com/assets/7018048/9149794/60f80c9c-3ded-11e5-8a56-7ae18ddc7a2f.png)

Author: wangfei <wangfei_hello@126.com>
Author: scwf <wangfei1@huawei.com>

Closes #7336 from scwf/dynamic-optimize-basedon-apachespark.