[Feature] Optimize `/content` add endpoints

[jcace]

• Calling POST /api/v1/datasets/contents/ (i.e, from edge-vertex) takes longer depending on how many contents are being published. In my testing, I saw it take several seconds with only ~20 contents to be added
• Additionally, importing from singularity where we had 8000+ contents took a solid 5+ minutes
• We need to optimize the db insert queries to speed it up - this should be a very fast operation. Currently it does each one individually to keep track of success/fail, so would be good to investigate if we can somehow batch them together for one db transaction instead

[jcace]

Some things we could do:

1. When matching up the collection names with the datasets, make a single query i.e select * from datasets; instead of a separate query for each unique collection name.
2. Put all of the db.Create() operations in a transaction https://gorm.io/docs/transactions.html , so they all execute at once