Add VerifyChannelUpgradeSequence method to 03-connection

[damiannolan]

Summary

The upgrade sequence MUST also have a verification method so that chains can prove the upgrade sequence on the counterparty for the given channel upgrade.

Using the existing state verification keeper functions in 03-connection for reference:

• Retrieve the ClientState and associated store using the provided connectionEnd
  • An error should be returned if the ClientState cannot be found
  • An error should be returned if the ClientState is inactive
• Construct the MerklePath using the UpgradeSequencePath defined in 24-host
• Invoke VerifyMembership using the associated ClientState

func (k Keeper) VerifyChannelUpgradeSequence(
	ctx sdk.Context,
	connection exported.ConnectionI,
	height exported.Height,
	proof []byte,
	portID,
	channelID string,
        sequence uint64,
) error {
        return nil
}

Ref: https://github.com/cosmos/ibc/blob/master/spec/core/ics-004-channel-and-packet-semantics/UPGRADES.md#upgrade-sequence-path

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Note the ClientState interface will contain a generic VerifyMembership method following the completion of:

• Add generic state verification functions to ClientState interface #1145

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