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 
@haoyuan 
I think expose channel for read/write in BlockHandlerLocal can meet the requirement. BlockHandler may support different under file systems. HDFS doesn't support channel. and remote read through data server is needed only for local file system