Are there open source RDBMS that support non-backup snapshots?
Arkhipov's extension for PostgreSQL enables Temporal Tables.

https://pgxn.org/dist/temporal_tables/

Here's a decent guide for using it: http://clarkdave.net/2015/02/historical-records-with-postgresql-and-temporal-tables-and-sql-2011/

PostgreSQL is uniquely suited to this task due to the way it implements row updates.