[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



Hopefully, rest of the code changes in this class is documented (a bit too heavily probably ? lighten it before final commit ?)