Once user calls freePath, remove block location information of the file from master immediately

currently when user calls freePath, Tachyon first delete block files on worker, and then delete location information on master. 
this may cause such issue:
if some user calls freePath to free some file, after that another user wants to read the file, but the location information is not deleted from master yet, then user will get corrupt location information of the block files. 

This change makes Tachyon first delete location information on master, and then delete block files on worker (act like delete). so that once some user calls freePath to free in memory blocks of some file / directory. other users can't get in memory locations of the file / directory any more.