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.
What's the performance of this? mLocalFileChannel.map(MapMode.READ_WRITE, inFileBytes, length) should a pretty high overhead. 

How about having a buffer, like the way https://github.com/amplab/tachyon/blob/master/core/src/main/java/tachyon/client/BlockOutStream.java is done.