refactor(x/provider): switch to epoch rewards

[julienrbrt]

Closes: #35

The other command in the issue has already been implemented by #55

[tbruyelle]

This change introduces an important shift in the debt system:

Before fees are distributed, the required amount is sent from the consumer fee pool to the provider module account. If this operation fails because of insufficient funds, the consumer chain becomes "in debt" and fees are not distributed.

With this PR, the required amount becomes much larger (multiplied by blocks_per_epoch), so there is more chance for a consumer chain to become "in debt" and to have the fees not distributed.

As long as blocks_per_epoch is small enough, this is probably fine, or maybe we should adjust the algorithm accordingly.

[julienrbrt]

With this PR, the required amount becomes much larger (multiplied by blocks_per_epoch), so there is more chance for a consumer chain to become "in debt" and to have the fees not distributed.

I don't get this. The amount does not change, it just gets paid in batches instead of per block.
As your 'rent' security, payment is made upfront so you will be declared in debt at the start of the epoch if it does not cover the whole epoch, but you would have anyway been in debt mid-epoch if we keep the current per block system.

[tbruyelle]

I don't get this. The amount does not change, it just gets paid in batches instead of per block.
As your 'rent' security, payment is made upfront so you will be declared in debt at the start of the epoch if it does not cover the whole epoch, but you would have anyway been in debt mid-epoch if we keep the current per block system.

Yes this is what I meant, a consumer chain can go "in debt" sooner than in the previous model. We can go with that, I just wanted to highlight it.