Merge hierarchy store into Tachyon [TACHYON-33 sub-task 5 & 6]

Merge hierarchy store into Tachyon, which supports storing data on both memory and other storage(SSD HDD etc), client side and RPC interface are also modified accordingly

JIRA:
https://tachyon.atlassian.net/browse/TACHYON-38
https://tachyon.atlassian.net/browse/TACHYON-39
the storageDir returned by requestSpace is only a top-tier storageDir, so this is should only return the top-tier storageDir.