limbo: fix commit/rollback failures with triggers

Currently some transactions on synchronous space fail to complete with
the `ER_CURSOR_NO_TRANSACTION` error, when on_rollback/on_commit triggers
are set.

This is caused due to the fact, that some rollback/commit triggers
require in_txn fiber variable to be set but it's not done when a
transaction is completed from the limbo. Callbacks, which are used to
work with iterators (`lbox_txn_pairs` and `lbox_txn_iterator_next`),
acquire tnx statements from the current transactions, but they cannot
do that, when this transaction is not assigned to the current fiber, so
`ER_CURSOR_NO_TRANSACTION` is thrown.

Let's assign in_txn variable when we complete transaction from the limbo.
Moreover, let's add assertions, which check whether in_txn() is correct,
in order to be sure, that `txn_complete_success/fail` always run with
in_txn set.

Closes #8505

NO_DOC=bugfix

(cherry picked from commit 6fadc8a0)