Dummy client: Allow for initialization at an earlier epoch

Right now the dummy client only initialize at the latest epoch. We need to update its logic to accept an initialization at any given epoch, so that it can ratchet its inner state to the latest one.

This will prove useful when we want to start the Light Client from genesis or when we want to test the client behavior over multiple epoch changes.

Start by locating the dummy client initialization logic and its epoch and inner-state handling. Trace how initialization currently selects the latest epoch, then verify behavior when starting from genesis or another earlier epoch and across multiple epoch changes.

Written by the indexing model from the issue text.