Meta server: fix user and group validation with chown rpc by returning an error in the case if the check fails. Return user and group ids and user and group name in chown rpc reply.
Client library: parse user and group name filed if present in chown RPC reply, and update user and group mappings accordingly. Implement user and group table update and lookup one entry "cache" by storing the last update and/or lookup iterators.
Qfs tool: always use user and group names / string versions instead of id versions. Change local file system to do user and group names to id mappings internally, inside local file system object method.