Implement box.begin(), box.commit(), box.rollback() in stored procedures

Right now any insert/update/delete a) commits the transaction b) is a yield point.

Implement explicit BEGIN/COMMIT/ROLLBACK words, which allow to start multi-update-statement transaction, commit it or roll it back.

These statements should be available inside stored procedures only. "Interactive" transactions,
i.e. a way to begin a transaction on behalf of a client, then issue statements, and then commit/rollback it, need not be supported.