Non-DYM gas fees handing in x/txfees

[keruch]

We handle AMM and IRO fees, but we're still left with gas fees. The current design is:

1. DYM fees go to staking rewards
2. Non-DYM fees go to x/txfees to be swapped and burned once an epoch

We need to unify the approach. The non-DYM fees are handled in the x/txfees AnteHandler. In theory, this should be simply changed for

txFeesKeeper.ChargeFeesFromPayer(ctx, acc.GetAddress(), fees, nil)

The problem is that we might have custom coins for fees (which are non-fee tokens). Currently such tokens are burned, but in case of changing the API, we're going to send them to the community pool. This potentially leads to the community pool spamming (as mentioned in the ADR). Though this statement is not proven and requires additional research.