feat: Fast Bridge unhappy path

[jaybuidl]

Resolves #53

[shotaronowhere]

Should use storage instead of memory to persist state changes in struct L94, L134, etc.

I fixed the registered mapping logic in the challenge-path-2-merkle-root branch. I think the single message fast bridge should implement a similar pattern for safe send.

Apologies, the fix I just pushed was supposed to be included in the original push, but I had 2 code editors open at the same time and on overwrote the other.

[jaybuidl]

Why close the PR thou..

[jaybuidl]

Should use storage instead of memory to persist state changes in struct L94, L134, etc.

Fixed

[shotaronowhere]

https://docs.google.com/document/d/1KYBEZ_sqUWPK92O7LPJaBuovoYy4J6UF5y5l74W0BsY/edit

[jaybuidl]

2. Unpredictable hashes using the block numbers.

Implemented

3. Challenger reward lower than the Bridger's one.

Can be done via configuration/governance.

4. Claims rate-limited to a period equivalent to the batching period.

Will be done in the branch feat/challenge-path-with-merkle-proof.

1. Null message to prove the non-existence of a ticketID.

Implemented this in a separate branch feat/challenge-path-2-null-message, it introduces extra complexity and edge cases which may not be necessary given 2. 3. and 4. .

[shotaronowhere]

3. Challenger reward lower than the Bridger's one.

Can be done via configuration/governance.

Governance / configuration can only change the challenger and claim deposit size. In order to change the pay-out of the challenger, we need to modify L170
To something like:

uint public constant ALPHA_DIVISOR = 1e4;
uint public rewardBurnRatio = 5e3; // half reward, half burnt

uint256 amount = ticket.claim.claimDeposit*rewardBurnRatio/ALPHA_DIVISOR + ticket.challenge.challengeDeposit;

And let governance update / change the rewardBurnRatio. Or to be honest, maybe rewardBurnRatio should be immutable.

To make sure that a dishonest claim -> self challenge does not result in a zero-cost delay grief.

[hrishibhat]

Looks good.

[codeclimate]

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