add storage dir to manage and store block files on different under file systems
some format change
add eviction strategy for swapping block files
some format change
update BlockHandler, change final variables and remove lastTier from interface
delete ufsConf when getting BlockHandler
modify according to comments
modify according to comments, add more synchronization control
add test cases for StorageDir
replace  dirIndex using StorageDir
make eviction stragey interface to accept the storage dirs candidate as input
modify code style
rename tachyon.worker.elimination to tachyon.worker.eviction
modify according to comments, use ConcurrentHashMap to replace HashMap
modify according to comments
modify according to comments, and reformat code style
modify according to comments
add test case for eviction strategy
remove deprecated TachyonFS.get() method
remove warnings