Make all `Client` methods to return structured values and error. Parse v2
status messages in all RPC and provide status getter from all result
structures. Returns status failures as status result instead of error.
Interface changes:
* all methods return `<method>Res` structure;
* rename some methods to be more clear;
* unify TZ and SHA256 objecy payload hashing in single method.
Behavior changes:
* client doesn't verify object header structure received via Object.Head.
If the caller was tied to verification, now it must do it explicitly.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
