API should return errors for violation of key version constraints

[dienummer]

I note the following deviations from the reference implementation's API contract/spec (https://github.com/lightningdevkit/vss-server/blob/main/app/src/main/proto/vss.proto) with respect to object PUTting

1. put object succeeds even if newly created item does not have version: 0
2. put object with lower version number than current should cause error (vss-rs accepts call and silently drops / does not update db)
3. put object with version field increasing by > 1 is accepted
4. missing delete_items field

I can tackle these; no promises on timeline

[benthecarman]

We cannot do 3 as that'd break mutiny. 2 seems sound but could cause problems. Otherwise 1 and 4 seem fine to me