Add StorageTier into Tachyon[TACHYON-33 sub-task 4]

The storage tier manages storage dirs, Every StorageTier points to its next StorageTier instance, unless it is the last level storage layer. new coming blocks always stores on the top tier, storage tier requests space and allocate storage dir for new coming blocks, if there is no enough available space, it will choose some old blocks to evict to its successor tier by certain eviction strategy, if current tier is the last tier, it will delete the old blocks
it can convert String format to enum via enum's static method valueOf