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box: use fake txn_stmt in space_before_replace

The test box-luatest/gh_8027_txn_handle_abort_in_trigger_test.lua fails
with an assertion failure:

```
./src/box/tuple.h:1508: tuple_unref: Assertion `tuple->local_refs >= 1' failed.
```

This happens because a before_replace trigger callback yields with
disabled memtx MVCC, which causes the transaction manager to rollback
the current statement in txn_on_yield screwing our assumptions about
stmt->new_tuple reference counting in space_before_replace.

We could take another reference to stmt->new_tuple to avoid that,
but modifying txn_current_stmt before it's initialized, as we do in
space_before_replace, looks unsafe so instead let's allocate a fake
statement on stack so as not to mess with the transaction manager
internals.

Closes #12779

NO_DOC=bug fix
NO_TEST=caught by existing tests

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