Validation about connection IDs for IBC client

[yito88]

IBC needs to store IBC connection IDs for each IBC client.
However, there is no validation about "clients/{identifier}/connections" for now.

IBC VP has to check if a new connection ID is stored for the corresponding client when the connection is created.

┆Issue is synchronized with this Asana task by Unito

[cwgoes]

@yito88 what is the status of this - have we done this already?

[yito88]

Aleks has worked on this in https://github.com/anoma/namada/tree/aleks/verify-ibc-connection-list. It hasn't been merged yet.
This issue is not fatal because the list is not used.
I'd like to merge it after updating ibc-rs (#840).

Only the IBC relayer would use the list. The current relayer can get it in another way.

[yito88]

Fixed by #1217