replicator: Do not bump repository generation for PackRefs

Similar in spirit to fc93593c (praefect: Stop creating replication jobs on cleanup, 2021-02-03), we're still bumping the repository generation when creating PackRefs jobs. But given that these only optimize the repository without changing the user-visible state of the repository, bumping the repository generation is only going to needlessly cause us to regard secondaries as out-of-date.

Fix the issue by introducing a separate "PackRefs" change type which doesn't cause us to bump the generation.