PCAs have to be sent (repeatedly) until corresponding parcels expire

[gnarea]

Executive summary

Due to an oversight, gateways have no option but to send repeated PCAs on each CRC synchronisation until the corresponding parcels expire. That's because there's currently no way for the receiving gateway to acknowledge the receipt of a PCA.

Describe the solution you'd like

One solution could be to introduce a new control message in the gateway called to signal that a PCA has been received. A PCA Acknowledgement (PCAA or PCA2). However, PCAAs would still have to be sent repeatedly unless we limit it to a small number of relays.