Merged branch 'database-level-autocommit'.

Fixed #2227: `atomic` supports nesting.
Fixed #6623: `commit_manually` is deprecated and `atomic` doesn't suffer from this defect.
Fixed #8320: the problem wasn't identified, but the legacy transaction management is deprecated.
Fixed #10744: the problem wasn't identified, but the legacy transaction management is deprecated.
Fixed #10813: since autocommit is enabled, it isn't necessary to rollback after errors any more.
Fixed #13742: savepoints are now implemented for SQLite.
Fixed #13870: transaction management in long running processes isn't a problem any more, and it's documented.
Fixed #14970: while it digresses on transaction management, this ticket essentially asks for autocommit on PostgreSQL.
Fixed #15694: `atomic` supports nesting.
Fixed #17887: autocommit makes it impossible for a connection to stay "idle of transaction".