Add Hierarchystore into Tachyon(Tachyon-33)

Add hierarchystore into Tachyon (details are on Tachyon's jira)

This feature aims to solve the problem by putting some data on memory and the rest data on high speed secondary storage devices with high capacity. the latest data will be putted into the top tier(the fastest), when top tier's storage space is full, it will swap the old data to the succession tier. when the data on succession tier is accessed again, it can be chosen to promote the data back to the top tier.

the work is divided into 6 sub-tasks, each sub-task is in one commit. 
level ++;

Please refer to the "code style" section in http://tachyon-project.org/master/Startup-Tasks-for-New-Contributors.html