Add storage dir and  eviction strategy [TACHYON-33][SubTask 2 & 3]

details are on Tachyon's JIRA
https://tachyon.atlassian.net/browse/TACHYON-33
storage dir is the container which manages and stores the data on different storage tiers. it also maintains the data migration between different storage dirs(on same or different storage tiers).
if current storage tier has no enough space for new coming block, some blocks needs to be swapped out to its successor storage tier, to get enough space. Evict strategy defines how to choose the blocks to be evicted. currently LRU based eviction strategies are implemented. an interface is also exposed for other eviction strategy implementation
Why is this a exception?  Can't that be encoded in the response by returning `null`?