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 
I have moved all members in BlockHandler to BlockHandlerLocal, but I still keep it an abstract class to have "get" method, which would make the other part of code follow the original design.