Improve Endpoint interface abstraction

[alfredo-stonk]

Endpoint interface for multihop proof generation

• add GetClientConsensusHeight
• remove GetClientState

Remove

• genClientProof since it's not used for proof generation. Can always call Endpoint.QueryProofAtHeight if it's ever needed again

Motivation: One client's ConsensusState height is normally the latest height of the clientState. But in cases of a local IBC connection (eg. Polymer's virtual IBC connections), this is no longer the case. GetClientConsensusHeight abstract away the actual logic of consensus height retrieval from the proof generation process. This change makes proof gen more robust for both remote and local ibc connections.

[dshiell]

I incorporated the non-conflicting changes from this PR after discussion with @7AC - I think we can close this out now.