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.
Because this merging thread is started before fetching shuffle data, so the size of `blocksAwaitingMerge` will be 0 if we haven't yet fetched even one block and this loop will directly quit.