box: forbid TCL statements in transactional triggers

User is not allowed to execute TCL statements in on_commit and
on_rollback triggers - it is documented UB. However, this prohibition
can be easily overseen: one could, for example, try to rollback current
transaction if something is not OK in on_commit trigger. So let's check
if TCL statements are not executed in transactional triggers.

The commit introduces a new helper `txn_check_can_complete` - it is
required because there are cases when transaction cannot be continued
(txn_check_can_continue will return error) but it's allowed to try to
commit or rollback it. Example: when MVCC aborts a transaction due to
conflict, the result is not observed by user. So he cannot execute new
statements in the transaction, but we cannot forbid to try to commit
the transaction - only then the error will be observed and the
transaction will be actually completed (rolled back due to conflict).
But after the attempt to commit the transaction, we must forbid to
try to complete it again - it will lead to UB.

Along the way, the commit removes unnecessary check in function
`box_txn_rollback`.

Closes #7331

NO_DOC=bugfix