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

resubmit the PR
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

old PR:https://github.com/amplab/tachyon/pull/550