ETag cache is not reset during backup load command

[Syndesi]

The backup load command expects an empty database, i.e. no nodes or relations, and then imports some data. While this does guarantee that no element data is present, cached ETags from previous usage can remain.

Tasks:

• [ ] Clear ETags during backup load command.
• [ ] Check if other cached data might need to be reset as well.
• [ ] Optionally create a new command / function which can clear all cached data, and rebuild caches if required.

[Syndesi]

Additional information: The endpoint GET / with no auth data, i.e. via anonymous access, is especially sensible to this bug, as the frequent healthcheck can result in generating an "empty ETag", which is then persisted within Redis, and not updated on further element updates.

The following tasks should be performed as well:

• [ ] Make sure that the ETag cache is being cleared before any elements are loaded.
• [ ] Do not generate new ETags until the whole backup is loaded. It might be also useful to block every API request while the backup is being loaded.
• [ ] Make sure that the ETag cache is still empty after the backup import was successful.
• [ ] Optional: Provide a new command to enable/disable API access easily.