SPARK-2532: Minimalistic shuffle consolidation fixes

All changes from this PR are by @mridulm and are drawn from his work in #1609.
This patch is intended to fix all major issues related to shuffle file consolidation
that @mridulm found, while minimizing changes to the code, with the hope that it may
be more easily merged into 1.1.

This patch is **not** intended as a replacement for #1609, which provides many
additional benefits, including fixes to ExternalAppendOnlyMap, improvements to
DiskBlockObjectWriter's API, and several new unit tests.

If it is feasible to merge #1609 for the 1.1 deadline, that is a preferable option.