DFs received by coordinator are unacked

Currently, coordinator fetches DF delta between local version and worker version. Coordinator sends it's local version as part of DF request. Worker is free to remove stored DFs based on coordinator local version. However, coordinator does not acknowledge received DFs until next DF is available. Therefore workers can keep DFs locally longer then needed.

This can be resolved by worker artificially bumping DFs version until there are still unacknowledged DFs