Fee management and incentive structure support

[AntoineRondelet]

Extend the project to support fee management and the implementation of incentive structures (around proof aggregation) on top of the protocol.
This is likely to require the extension of existing data structures (see: b61986e the int32 fee_in_wei = 3; attribute of the RPC message TransactionToAggregate for instance).

Take a more general approach to this to make the project more generic with regard to incentive structures support.