txn: pass txn_stmt instead of txn to on_commit/on_rollback

Currently on_rollback triggers are called on rollback of the whole
transaction. To make it possible to invoke them on rollback to a
savepoint, we need to pass a statement at which the savepoint was
created.

Needed for #9340

NO_DOC=refactoring
NO_TEST=refactroring
NO_CHANGELOG=refactoring

(cherry picked from commit a1d85827)