[2.6.x] Concurrent artifact creation test & fixes

[EricWittmann]

This PR fixes a concurrency problem in the SQL storage when creating artifacts. There are several "upsert" style operations that must happen when creating artifacts, including:

1. Creating the Group if one does not already exist
2. Inserting Content only if it does not already exist
3. Incrementing sequences (e.g. globalId and contentId)

There were existing race conditions (database dependent) for some or all of these operations.

This PR changes 1 and 2 so that the "Ensure Group Exists" and "Ensure Content Exists" operations occur in their own transactions. This allows us to forgo using UPSERT and instead simply try the INSERT and react to the primary key constraint violation. If a PK violation occurs then we simply go on to the next part of the create (e.g. if creating the group fails, it means the group already exists and we can skip to the next thing).

This new approach fixes the concurrency issues and also simplifies the logic a little bit. But it does this at the expense of introducing a small chance that a Group will be created even though it is not needed (e.g. if multiple concurrent operations attempt to auto-create the group but then all fail). The same is true of content.

As a result, this change MAY result in empty Groups or orphaned content. Orphaned content will get cleaned up automatically, but empty groups would need to be deleted manually. I think this is an acceptable tradeoff.