After all data is sent, the log item is saved to database and response is returned.
The value database field is either log contents or blob holder, depending on persistence method
determined when data was uploaded.

Depends on D6213

The endpoint can be tested manually (e.g. BloomRPC) or by yarn run-integration-tests backup - these will fail yet, but some logs should be created, which can be checked by e.g. looking at backup and blob services logs or using aws-cli.