Limit LOCK members number to one

[carpawell]

Is your feature request related to a problem? Please describe.

Same as in #311 but for LOCK objects. It is a little bit harder because there is no LOCK RPC and every LOCK object is created from scratch on the client side currently.

Describe the solution you'd like

It can also be restricted to one LOCK -- one LOCKed object. This also makes it more flexible for expiration and other side effects while LOCKing.

Describe alternatives you've considered

Keep it as is.

Additional context

Uncontrollable number of locked objects by a single object makes it harder to maintain object meta information: nspcc-dev/neofs-contract#448 (comment).