"Invalidate" sent transaction if it has not been verificated in 30 minutes

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In this implementation transactions are not included in the block but replaced with transaction id. There should be a timeout for a transaction to get its first verification. If it is older than that, it should be retransmitted with the block it belongs. For practical reasons, timeout should be longer for 10 minutes (average block interval) but shorter than 30 minutes (you must wait this long before receiving a valid blockchain).