[lab3-part1-step2] complete function rpcs::checkduplicate_and_update
[lab3-part1-step2] complete function rpcs::add_reply
[lab3-part1-done] fix stupid bugs:
		  1. iterator forgot to increase
		  2. dynamic array forgot to malloc
[lab3-par2-done] complete a simple prototype of the distributed file system.
[lab3-part2-fix] fix wrong rpc calls.
[lab3-part3-fix] fix bugs from previcous labs yfs_client
[lab3-part3-done] implement locking to ensure the atomicity of file system operations when there are multiple yfs_client processes sharing a file system.