Implement fishing mode

[seunlanlege]

ref: #76

Should also add to testsuite

• client freezing semantics via misbehavior
• client upgrades

[seunlanlege]

Thought about the design for this and unfortunately, there's no way to share code between clients. So instead clients are free to implement this however they like but have to expose a long running future that drives the fisherman task.

Fisherman Algorithm:

• We watch the chain for all UpdateClient events emitted for the client we're watching and query the transaction that updated the client. This transaction contains the finality proof.

• On obtaining the finality proof. We query a full node for the finality proof at the updated height and check if the header matches our full node.

  • Full nodes will reject invalid state transitions and will also keep forks letting us know a header is being disputed.

• If we do not detect forks and the finality proof matches the light client update, repeat from start

• Otherwise construct Misbehavior for the light client.

• Construct an equivocation report for the source chain.

• Repeat from start.