SerializedShuffleHandle is a ShuffleHandle to identify the choice of a serialized shuffle.
SerializedShuffleHandle is created when SortShuffleManager is requested for a ShuffleHandle (for a ShuffleDependency). SortShuffleManager determines what shuffle handle to use by first checking out the requirements of BypassMergeSortShuffleHandle before SerializedShuffleHandle's.
SerializedShuffleHandle is used to create an UnsafeShuffleWriter.
SerializedShuffleHandle takes the following to be created: