Added a number of very fast, memory-efficient data structures: BitSet, OpenHashSet, OpenHashMap, PrimitiveKeyOpenHashMap.
Eliminate extra memory usage when shuffle file consolidation is disabled

Otherwise, we see SPARK-946 even when shuffle file consolidation is disabled.
Fixing SPARK-946 is still forthcoming.
Merge commit '3e7df8f6c6edec9e71c6e416de86aa1a2cefc176' into shuffle-fix
Use flag instead of name check.