Add in helper vector with transactionsId already send triggered by tansaction_queue

[Matthias-NIDEC]

Describe your changes

In the case a charging process is ongping, the conenction to the backend is disconnected and the charging process is stopped.
It is stored inside the transaction databse with csms_ack=0 and the stoptransaction.req is stored inside the transaction_queue.

On boot up, the transaction_queue is emptied and the transactions with csms_ack=0 are resend. If the message_queue is alredy emptied before the check of the open transcations, then the stopTransaction.req is sent again.

In order to avoid that a helper vector with transacitonids in flight is added.

Issue ticket number and link

#907

Checklist before requesting a review

• I have performed a self-review of my code
• I have made corresponding changes to the documentation
• If OCPP 2.0.1: I have updated the OCPP 2.0.1 status document
• I read the contribution documentation and made sure that my changes meet its requirements