add block handler to handle block files on different under file systems[Tachyon-33][SubTask 1]

details are on Tachyon's JIRA
BlockHandler is used to read and write block files on different under file systems, currently local file system is realized.
because if BlockHandler is only used for reading, it is not necessary to set full permissions. the operation will cost about 50 ms in my testing, which will be a  huge cost when block files are very small. 
write and delete need to set permission.