Reward proposers for including votes and reaching finalization

[Gnappuraz]

As described in [1] we want to reward proposers substantially only if finalization is accomplished and in a proportional manner in respect of how many votes were included.

In order to do so we need to change the coinbase structure:

• proposers will get only a small fraction of the block reward immediately, and all the fees. (#794)
• proposers will get the remaining fraction as their block gets finalized. This amount should be proportional to the amount of total votes that were included in this epoch as described in [1]. This reward will be also considered mature since the relative block is already more than 100 blocks behind the tip.

As a note, it becomes possible that, in case of non-optimal finalization, we are gonna need to reward in one single block, many hundreds of proposers that are part of the last dynasty.