Note on the AFF4 datastore deprecation

Starting from the version GRR uses a new datastore format by default - REL_DB. REL_DB is backwards-incompatible with the now-deprecated AFF4 datastore format (even though they both use MySQL as a backend).

Use of AFF4-based deployments is now discouraged. REL_DB is expected to be much more stable and performant. Please see these docs if you’re upgrading an older GRR version and would like to try out the new datastore.