add spark.shuffle.spill.batchSize and fix the value of spark.shuffle.manager

```scala
private val serializerBatchSize = sparkConf.getLong("spark.shuffle.spill.batchSize", 10000)
```
add `spark.shuffle.spill.batchSize` to doc.

And according to 
```scala
    // Let the user specify short names for shuffle managers
    val shortShuffleMgrNames = Map(
      "hash" -> "org.apache.spark.shuffle.hash.HashShuffleManager",
      "sort" -> "org.apache.spark.shuffle.sort.SortShuffleManager")
    val shuffleMgrName = conf.get("spark.shuffle.manager", "hash")
```
value should be "hash" and "sort" rather than "HASH" and "SORT"