distributions: implement pool budget

[erwanor]

The distribution component defines an EpochHandler extension trait, with an end_epoch method. This handler is triggered at the end of an epoch and executes the following logic:

1. Compute the size of the tournament incentive pool for the ending epoch:

let epoch_duration = epoch.end - epoch.start;
let lqt_block_reward_rate = self.get_lqt_block_reward_rate();
let total_pool_size = lqt_block_reward_rate * epoch_duration;

The pool size expands or contract based on how long the epoch lasts. Since epochs are at most ~2 days, this gives a predictable upper bound on the total issued rewards. On the other hand, since the pool size contracts with epoch length, this means that there’s no upside to trying to make the chain churn epochs.