Limit TOMBSTONE members number to one

[carpawell]

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

I'm always frustrated when there is only a single object in the DELETE RPC but a lot of objects are in the tombstone's body. It is even more unexpected after #297 (however, it is not that clear when we are cleaning unfinished big objects).

Describe the solution you'd like

Disallow "big" tombstones. One TS -- one object. Tombstones expire sooner or later and tombstones with many objects are not expected, it is even not possible currently due to DELETE RPC, only if someone creates it manually.

Describe alternatives you've considered

Keep it as is.

Additional context

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

[cthulhu-rider]

rly desired but very breakin change, lets do asap. #312 too

[roman-khimov]

It also allows to do placement by the original object OID in future making auxiliary objects follow original ones with higher probability (reducing negative effects from things getting out of sync).