Split 'content_add' method into 'content_add' and 'skipped_content_add'.
Respectively to add present content and skipped content.
This simplifies the logic of both methods, and is a necessary step to typing / using swh-model objects everywhere, as contents have quite different attributes depending on whether they are present or missing.
For the moment, this diff only implements for the in-mem backend, not pg nor cql
Migrated from D2626 (view on Phabricator)