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.
Some initial letter in code comment are capitalized, but some are not. Please keep the style consistent as the Tachyon's style. Make the 'close' be 'Close' here. Also, please check through this PR  and the following PRs.