Client lib: update user and group mapping cache when changing table entries. Do not reset user and group names and ids, in the case if RPC status indicates an error. Make chunk server RPCs (size, get chunk meta data, and read verify) initiated from withing the KfsClientImpl class work with chunk access / security enabled. This is needed in order to make all "utility" methods such as chunk checksum, replicas size verification, and non rs files computation in the cases where the meta server doesn't return the file size in the file attribute, work with "security" enabled. With security enable obtain chunk read lease, and extract the chunk server and chunk access tokens from the lease acquire response, before issuing the corresponding chunk server rpcs.