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.
Ah thanks, but I meant that since we now only support local block handler, we could just do check for it:

public static BlockHandler get(String path, Object ufsConf) throws IOException, 
    IllegalArgumentException {
  if (path.startsWith(Constants.PATH_SEPARATOR) || path.startsWith("file://")) {
    return new BlockHandlerLocalFS(path);
  }
   throw new IllegalArgumentException("Unsupported block file path: " + path);
}

No need to check for path for UnderFileSystem.isHadoopUnderFS