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

Add a new cli to free in memory file/folders
This maybe the reason: free() call goes to the master node. master takes sometime to make it happen in the worker node. When you did it manually, two commands may take more than 1 sec, which is the heartbeat interval. When you tested it using unit test, the interval of two calls is far less than 1 sec.

One way to figure it out is to try to trace the code path for this. This can also help understand the code base better.