Update replace_producer_keys for instant finality

[heifner]

Needs to include updating the finalizer keys along with the new proposer schedule (active_proposer_policy).

[arhag]

There is no need to complete this before Leap 6.

[bhazzard]

The use case of this is to be able to purposely fork mainnet, which could be useful for certain testing and experimentation scenarios.

An alternative approach could be a tool that could allow a user to make modifications to a snapshot. The spring-util tool approach may be preferred because it avoids the complexity of conditional behavior based on what mode a node is running in.