Mock testing for ICS 18 (relayer) with update client messages

[adizere]

Summary

Towards obtaining a production relayer, we plan to start off with a simple logic (following in the footsteps of the relayer algorithm for ICS18 but perhaps simpler) that handles the relaying of client update messages between two mocked chains. The goal is to obtain a "ping pong" of client updates between the chains.

This issue can be considered the "local" part of informalsystems/hermes#216.

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