Support for DBMS Upsert expressions

[sandro-salles]

Hey Wesley!

Nice work done here, congratulations...!

I was starting to write a code that is somewhat similar to the bulk_upsert function from your code, but relying on the native DBMS UPSERT implementations (PostgreSQL 9.5 specially), where instead of two separate queries (bulk_create and bulk_update), the whole operation would happen on a single upsert query.

Do you sometime have considered using it? I think this would be a great performance improvement for the Django ORM!

As soon as i have a more stable version of my code i'll let you know so you can maybe collaborate!