Add a migration to ensure all repositories registers their paths to partition's KV

Each partition has a relative paths mapping for the repositories it hosts. But currently this is only added when a new repository is created. Existing repositories that are migrated to partitions are missing these mappings.

When we start a transaction against an entire partition without a specific relative path, we use these relative paths and only include those in the snapshot. Therefore taking a backup of this snapshot would miss any other repository that is in the partition but missing its path mapping.