Support UPSERT

[roshan]

A common pattern is "Create if this doesn't exist, otherwise update". The conflict is determined as being on a duplicate key. Upsert is called:

• INSERT ... ON CONFLICT UPDATE in PostgreSQL
• INSERT ... ON DUPLICATE KEY UPDATE in MariaDB
• INSERT ... ON CONFLICT DO UPDATE in Sqlite

[roshan]

@tuliren bump

[pratikmallya]

Need this yesterday

[tuliren]

@roshan, thank you for the suggestion. Would you like to give it a try and create a PR? 😏