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.
The error message could be changed to "Unsupported block file path: " +path. This way we do not have to separate error message between HDFS and other schemes.