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.
@dcapwell 
there are two cases that needs check permission.
1. if there are more than one user, the block file is created by some user, and deleted by another user.
2. if block data is stored on persistence storage, and copied/backup to some new machine by other user, when deleting the added blocks on the new machines, permission needs to be checked.