[Deliverable] RLN Relay Incentivisation - Distribute RLN fees PoC

[chair28980]

Proof of concept mechanism for distributing RLN membership fees to RLN relay nodes based on an on-chain consensus mechanism for routed messages. Publishers over an RLN-controlled network pay membership fees, which entitle them to send a given amount of messages per unit of time while the membership remains active. Currently these membership fees are not used and are simply returned on membership expiry. These fees can be used to incentivise the RLN relay nodes that support the rate-limited infrastructure by routing and validating messages.