Checkpoint: Implicit execution of top-down checkpoints

[adlrocha]

Related: #103

Top-down checkpoints are used to agree on the state of the parent that the validators of a subnet have seen, so they can agree when a top-down message is final, and they can trigger the execution of top-down messages. Currently, top-down checkpoints are submitted in the same way as bottom-up checkpoints, through explicit messages from a validators. However, in order to remove the need from gas from validators and all the gas-related problems in the submission and execution of top-down messages, we can probably execute the submission of top-down checkpoints implicitly. With this:

• We will be able to remove the checkpointing period, and we will be able to submit a top-down checkpoint for every epoch as if it was a cron.
• We would remove the need of all the IPC agent-specific off-chain logic for top-down checkpoint submission.
• We would simplify a lot of the processes in Lotus and the agent.