In effect, allows to write all but the colliding contents to the storage. This
is the same behavior currently existing in the journal replayer (D2800). This
shares the behavior within the retry proxy as this is a common need, for
example in loaders as well (D2973).
And the colliding hashes are stored in the log in a formatted way so we do not
lose that information.
Related to D2800#66985
Related to D2973#72387
TODO:
- add tests for content_add_metadata and skipped_content_add