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
toRequestSpaceBytes doesn't always equal to requestSpaceBytes, it may equal to mUserQuotaUnitBytes.

for the logic availableSpaceBytes != null,  it checks whether there is any available space record in mIdToAvailableSpaceBytes for the storageDirId, if there is not any, availableSpaceBytes equals toRequestSpaceBytes, else availableSpaceBytes plus toRequestSpaceBytes as new availableSpaceBytes