tx: set session and user for commit/rollback triggers

Commit/rollback triggers are run asynchronously, upon receiving the
write status from WAL. We can't run them in the original fiber that
submitted the WAL request, because it would open a time window between
writing a transaction to WAL and committing it in tx, which could lead
to violating the cascading rolback principles. As a result,
commit/rollback triggers run with admin privileges.

Let's fix this issue by temporarily setting session and credentials to
the original fiber's for running commit/rollback triggers.

Closes #7005

NO_DOC=bugfix