[Consensus] Penalize validators for double signing

[Olshansk]

Objective

Penalize validators that double-signed competing state transitions.

Origin Document

This was discussed during a core team protocol hour. Due to Pocket's application-specific stack, we can eliminate the need for Tendermint's Evidence in exchange for a simpler solution.

Some of the ideas discussed included looking at getQuorumCertificate and shown in the comments below:

func (m *consensusModule) getQuorumCertificate(height uint64, step typesCons.HotstuffStep, round uint64) (*typesCons.QuorumCertificate, error) {
	var pss []*typesCons.PartialSignature
	for _, msg := range m.MessagePool[step] {
	  // ...

	hasDoubleSign :=  /* pseudo-code: group by msg.SenderAddress + check for multiplicity > 1 */
    if hasDoubleSign {
       // Maybe: Send a message to award the validator who caught double sign
       // Send message to slash the signer that did a double sign
    }

	  // ...
	return &typesCons.QuorumCertificate{
		Height:             m.Height,
		Step:               step,
		Round:              m.Round,
		Block:              m.Block,
		ThresholdSignature: thresholdSig,
	}, nil
}

Goals

• Identify how double signing can happen and how it should be handled in Pocket Network v1
• Identify other missing elements in the codebase that need to be accounted for in lieu of Tendermint's evidence

Deliverable

• [ ] Implementation that penalizes validators for double-signing state transitions
• [ ] Sufficient tests for the implementation above
• [ ] Add a new README explaining how double signing attacks are handled
• [ ] Update the appropriate CHANGELOG(s)
• [ ] Creating Github issues for subsequent work that is handled by Tendermint's evidence and is not done in Pocket network V1

Non-goals / Non-deliverables

• Reimplenting everything that Evidence does in Tendermint

Testing Methodology

• [ ] make target for a new set of tests
• [ ] All tests: make test_all
• [ ] LocalNet: verify a LocalNet is still functioning correctly by following the instructions at docs/development/README.md

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Creator: @Olshansk Co-Owners: @gokutheengineer