Wire the epoch transition listener into the instance

[samliok]

Adds epochTransitionListener and hooks it into the instance:

• NewInstance constructs the listener and wires its onIndex into both the validator and non-validator storage paths (replacing the no-op hook).
• When a transition block is indexed, the listener either generates and broadcasts auxiliary info, or once the history is sufficient, signs and broadcasts an epoch transition approval. Validators also record their own approval locally; non-validators pass a nil handler.
• HandleMessage routes incoming AuxiliaryInfo and EpochTransitionApproval messages into the MSM.