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
Won't this effectively get space from a random directory? Wouldn't it be better to try the available directories tier by tier, starting with memory?