Implement checkpoint validation in the Gateway actor

[aakoshh]

Implement the validation of bottom-up checkpoints in the IPC Gateway actor.

That checkpoint validation in the actor should include:

• Validating the signatures - doing this in the interpreter can be cheaper
• Checking that all signatories are part of the currently expected validator set
• Checking that the total power of the signatories is larger than 2/3
• Checking that the height of the checkpoint is the expected one
• Checking that the next validator set ID is known
• Remembering the relayer to be rewarded later, once we know how many relayers have completed the job.

[dnkolegov]

Implemented. The following PR is the final part consensus-shipyard/ipc-solidity-actors#229