txn: rework txn_on_yield trigger

We are going to implement timeout for transaction, after which it
will be rolled back. The timeout starts counting from the moment of
the first yield. Previously `txn_on_yield` trigger installed only
in case when engine does not support yields for transactions. Now it
is installed when transaction is created, but if transaction supports
yields it doesn't do anything. In the following patches, we will use
this trigger to start the transaction timeout countdown.

Part of #6177