limbo: set user for triggers on sync transaction

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.

The issue was already solved for confirming async transaction, but
session and user are still not correct, when the transaction is
confirmed by the limbo. Let's fix this issue by temporarily setting
session and credentials to the original fiberfor running
commit/rollback triggers.

Closes #8742

NO_DOC=bugfix