[SPARK-20950][CORE] add a new config to diskWriteBufferSize which is hard coded before

## What changes were proposed in this pull request?

This PR Improvement in two:
1.With spark.shuffle.spill.diskWriteBufferSize configure diskWriteBufferSize of ShuffleExternalSorter.
    when change the size of the diskWriteBufferSize to test `forceSorterToSpill`
    The average performance of running 10 times is as follows:(their unit is MS).
```
diskWriteBufferSize:       1M    512K    256K    128K    64K    32K    16K    8K    4K
---------------------------------------------------------------------------------------
RecordSize = 2.5M          742   722     694     686     667    668    671    669   683
RecordSize = 1M            294   293     292     287     283    285    281    279   285
```

2.Remove outputBufferSizeInBytes and inputBufferSizeInBytes to initialize in mergeSpillsWithFileStream function.

## How was this patch tested?
The unit test.

Author: caoxuewen <cao.xuewen@zte.com.cn>

Closes #18174 from heary-cao/buffersize.