Unnest to speed up inserts

[jonatas]

Hi @jamis, I work at Timescale and just see this new research from a colleague showing some performance gains using unnest for the sql statement.

https://www.timescale.com/blog/benchmarking-postgresql-batch-ingest/

The django folks are going to also implement, and I thought about suggesting to Rails, but it seems the perfect place still the bulk_insert gem instead. WDYT?

[jamis]

@mberlanda has been doing more on this project than I have (thank you, Mauro!), but honestly, with the addition of insert_all in Rails 6 (https://api.rubyonrails.org/classes/ActiveRecord/Relation.html#method-i-insert_all) there may be less value in this bulk_insert library than there used to be. Personally, I'm no longer working on bulk_insert, so I'll defer to Mauro on this.