Tachyon-74 : Add free of in-memory file/folder from CLI

Add a new cli to free in memory file/folders
@haoyuan : Thanks for pointer. I didn't realize, about the heartbeat interval. You are absolutely right. I re-tested with introducing CommonUtils.sleepMs(null, Constants.SECOND_MS * 2) and unit tests passed. Introducing sleep of 2secs okay in test case? Should I update the PR?