[SPARK-2532] Consolidated shuffle fixes

Status of the PR
- [X] Cherry pick and merge changes from internal branch to spark master
- [X] Remove WIP comments and 2G branch references.
- [X] Tests for BlockObjectWriter
- [x] Tests for ExternalAppendOnlyMap
- [x] Tests for ShuffleBlockManager



Nit: no space after assert (it's not a keyword)