refund fees which cannot be distributed

[colin-axner]

Summary

In ICS29 it is possible the forward relaying address provided is invalid (or perhaps a blocked address). This causes the forward relaying fee to be left in escrow.

We should consider doing something with the forward relaying fees. Potential options:

• refund
• send to community pool

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[crodriguezvega]

I see that the ICS-29 spec says the following:

If the relayer does not register their counterparty address, or registers an invalid address; the acknowledgment will still be received and processed but the forward fee will be refunded to the original fee payer(s).

[damiannolan]

I think this issue is mostly done. See: https://github.com/cosmos/ibc-go/blob/ics29-fee-middleware/modules/apps/29-fee/keeper/escrow.go#L58

We can also add a check for the block addrs.

• Add BlockedAddr(sdk.AccAddress) bool to expected BankKeeper interface
• Add logical AND (&&) with !k.BankKeeper.BlockedAddr(forwardAddr) to conditional