[Tachyon-14] TachyonURI

Merge request is just an update of the TachyonURI class from the uri branch. There is no functional change as it is not being used currently. I plan to submit the PRs for using it throughout the codebase incrementally.
The logic here is different from hadoop.fs.Path. We replace the \\s regardless of if its a windows path or not. Also I change the order of the replaces to address paths like C:\\\\a (this was a todo before). However, I don't know if that should be treated as a valid path.