Add spring-util capability to replace proposer and finalizer keys

[heifner]

Controller has two existing methods: replace_producer_keys and replace_account_keys.

• replace_producer_keys currently does nothing under savanna.
• Consider adding a replace_finalizer_keys.
• Consider adding a spring-util which would allow replacing the producer and eosio account keys for a legacy chain.
  • Under Savanna this option would also replace the finalizer keys
• Consider always changing the chain_id or providing a mechanism to change it.
  • Note changing the chain_id would also require updating the chain id in the block log if block log support was provided.
• Likely this capability should modify a snapshot updating the keys.
  • Updating snapshot by-passes issues with a fork switch undoing the key replacements.

[bhazzard]

This is a good idea, but we should wait until we actually need to use this functionality before implementing.