[Prover] ProvingBroker resolves conflicts when same jobs is worked on by two agents

[alexghr]

In case two agents are working on the same job, the broker must be able to resolve conflicts:

• it should accept the first proof that it receives
• it should inform the other agent that the job is no longer available the next time the agent checks in

The only way the system should end up in this situation is if the broker goes down while a proof is in progress and when the broker comes back online it hands off the same request to a different agent.