Enable the Cortina network upgrade:

X-chain

• Prevent vacuous acceptance of vertices in the Avalanche
• Introduce non-whitelisted conflict support to Snowstorm
• Introduce StopVertex serialization support
• Introduce StopVertex verification support
• Convert P2P network to use protobuf
• Modularize the AVM similarly to the PlatformVM
• Add EngineType to bootstrapping and consensus request messages
• Dispatch P2P messages based on the requested EngineType
• Introduce Block verification and execution to the AVM
• Shutdown the Avalanche engine after the StopVertex is accepted
• Transition to the Snowman engine after the StopVertex is accepted
• Wrap the AVM by the ProposerVM inside the Snowman engine

P-chain

• Add DB support for tracking accrued delegation rewards for validators
• Batch delegation rewards for validators whose StartTime is after the Cortina activation

C-chain

• Increase individual block Gas Limit to support more complex transactions