Previous identity version contains redundant entries

[maxtropets]

It seems like previous_service_identity_version was created to maintain a chain between service identities changes caused by DR, at least it follows from the name.

However, it contains redundant entries, which point to the same service identity. At first glance it's unclear why, because it's only supposed to happen on open_service. This needs to be investigated.

The only things which relies on this table is populate_service_endorsements, which still works but makes unnecessary key verification attempts with the same key, when jumping through the same key entries.

However, the field is also exposed at a part of service table, so may be read by users.