Refinement of FailureReason and TxStatus according to the latest opinions

[bertllll]

Newly emerged actualities from the development work on the functionality of the PendingPayableScanner, the core processes of managing the SentTx structures for still active, pending txs and FailedTxs that represent basically anything that could go wrong with the tx, led to a design talk which produced a more realistic working paradigm, as the former understanding lacked some important inputs that needed to be included later. This is a natural process of the development.

Aside the main gravity being in the firstly mentioned scanner, and what will result in immediate changes within this card, there is also a conclusion to remember regarding the PayableScanner:
In the retry payable mode, when operating the start method, it selects those failed_payable records which say failure_reason = RetryRequired, nad when done reading, it also needs to change the state into the other value, Concluded. So, this is purely the responsibility of this scanner, not the PendingPayableScanner!