Allow ics20 to use multiple ports

[ethanfrey]

Tokens with the same denom and same source chain, but coming from different ports are already distinguished in the current cosmos-sdk implementation (and I assume in the spec), thus limiting it to only one port on the other side is no longer needed for security.

For CosmWasm, we have a cw20 token contract (similar to erc20) and would like to implement ics20, so it can send tokens over ibc to another chain and redeem them (just one denom). We could require CosmWasm contracts on both sides with a custom protocol, but ics20 should be capable of this.

There would clearly be a difference of the uatom sent from transfer port vs. the uatom sent from wasm-cosmos1y3x7q772u8s25c5zve949fhanrhvmtnu484l8z.

I would request a change to the spec, and then we can look into realizing that in the Cosmos SDK.

[ethanfrey]

@cwgoes this is what we discussed yesterday

[cwgoes]

Aye, thanks for opening the issue. ICS 20 should be able to support this without trouble, since the denominations are already distinguished by the combination of port & channel identifiers.