Investigate missing gRPC APIs to allow efficient relaying of incentivized packets

[soareschen]

Summary

This issue tracks the outstanding GRPC APIs that are required for the relayer to efficiently perform relaying on incentivized packets. Feedback needs to be given to cosmos/ibc-go#276 before it is merged. The following APIs are missing:

Query Counterparty Address (cosmos/ibc-go#1132)

The relayer needs to be able to check whether it has already registered its counterparty address for a specific channel. Otherwise it can only blindly re-register its counterparty address before relaying which wastes transaction fees.

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[seanchen1991]

Closing this issue as 'done' since the gRPC APIs in question have been merged on the ibc-go side.