## What changes were proposed in this pull request?
When netty transfer data that is not `FileRegion`, data will be in format of `ByteBuf`, If the data is large, there will occur significant performance issue because there is memory copy underlying in `sun.nio.ch.IOUtil.write`, the CPU is 100% used, and network is very low.
In this PR, if data size is large, we will split it into small chunks to call `WritableByteChannel.write()`, so that avoid wasting of memory copy. Because the data can't be written within a single write, and it will call `transferTo` multiple times.
## How was this patch tested?
Spark unit test and manual test.
Manual test:
`sc.parallelize(Array(1,2,3),3).mapPartitions(a=>Array(new Array[Double](1024 * 1024 * 50)).iterator).reduce((a,b)=> a).length`
For more details, please refer to [SPARK-14290](https://issues.apache.org/jira/browse/SPARK-14290)
Author: Zhang, Liye <liye.zhang@intel.com>
Closes#12083 from liyezhang556520/spark-14290.
c4bb02abf2