Remove caching of checkpoints from the Inbox

[asaj]

This PR removes cached checkpoints from Inbox. Instead, checkpoints are expected to be verified in the same transaction as message processing.

Rationale:

1. With the move to fraud proofs that require merkle proofs in Only cache Outbox checkpoints for fraud proofs #475, there is a vulnerability with cached checkpoints where validators cache a fraudulent checkpoint but do not attempt to process messages against it until they're un-bonded. This is easily mitigated by requiring a proof against that checkpoint, but that is counter-productive to the idea of cached checkpoints as a way to save gas.
2. Removing the notion of cached checkpoints reduces complexity of the off-chain agents.
3. Gas savings can still be achieved by batch-processing multiple messages against the same checkpoint.

Other changes:

• Remove leafIndex and proof from the Inbox.Process event, and the unused InboxValidatorManager.Quorum event. These were included to allow fraud proofs to be constructed more easily, but it doesn't make sense to incur the extra gas costs until validators are bonded
• Rename BadRecipientHandle to BadRecipient2 for consistency

[yorhodes]

sweet, so many net red changes lately 🧨