Readability improvements to SortShuffleReader

Some of the changes include:
* Changed ioSortFactor to maxMergeWidth to clarify what it means.
* Changed "partition" to "block" to avoid confusion with Spark's more typical use of the word partition.
* Added header documentation.
* Rearranged and documented the main merge loop.
In that case we'd end up spilling immediately (further down), which is the correct behavior, right?