Fixed #21239 -- Maintained atomicity when closing the connection.

Refs #15802 -- Reverted #7c657b24 as BaseDatabaseWrapper.close() now
has a proper "finally" clause that may need to preserve self.connection.
Fixed #21202 -- Maintained atomicity when the server disconnects.

Thanks intgr for the report.

This commit doesn't include a test because I don't know how to emulate a
database disconnection in a cross-database compatible way.