[SPARK-6479] [BLOCK MANAGER] Create off-heap block storage API

This is the classes for creating off-heap block storage API. It also includes the migration for Tachyon. The diff seems to be big, but it mainly just rename tachyon to offheap. New implementation for hdfs will be submit for review in spark-6112.

Author: Zhan Zhang <zhazhan@gmail.com>

Closes #5430 from zhzhan/SPARK-6479 and squashes the following commits:

60acd84 [Zhan Zhang] minor change to kickoff the test
12f54c9 [Zhan Zhang] solve merge conflicts
a54132c [Zhan Zhang] solve review comments
ffb8e00 [Zhan Zhang] rebase to sparkcontext change
6e121e0 [Zhan Zhang] resolve review comments and restructure blockmanasger code
a7aed6c [Zhan Zhang] add Tachyon migration code
186de31 [Zhan Zhang] initial commit for off-heap block storage api