Define LOCK object type

[alexvanin]

LOCK object type is an object type that restricts object removal. Its payload should be a list of objects that should not be deleted.
If user sends object.Delete request on a object which is in this list, then it should return error.

Question: should object.Put(tombstone) also search for LOCK objects? Or metabase may deny tombstones if there is a LOCK object already /cc @realloc

If Storage node's GC wants to remove an object (due to expiration epoch, for example), it should not do that if LOCK object is present.

LOCK objects distributed over all container nodes the same way as tombstones.