Add NewHeaderBlock event to MockCheckpointManager and ICheckpointManager

[bogdan]

Some operations require us to track NewHeaderBlock event of RootChain contracts. That is possible to do. However, the functionality can not be verified in the test environment because MockCheckpointManger doesn't emit this event.

Please add this event to ICheckpointManager and emit it inside MockCheckpointManager#setCheckpoint to make it more consistent with RootChain contract.