[TASK] Ignore the vote_result of subsequent signing sets if one signing set has successfully signed proposal

[1xstj]

Task summary
In the current situation, we process a proposal to be signed by multiple signing sets for redundancy. This ensures that atleast one signing set can succesfully sign a proposal in the worst case. But in the best case all the signing sets will sign the proposal and store in the offchain storage. This will break the proposal handler verification since we submit multiple signed versions of the same proposals.

In #577 we have taken the simpler approach of not adding a signed proposal to offchain storage if it exists and is already signed by another signing set. We could improve this so that when a proposal with a proposal_hash is signed we can abandon all the other threads that are in the process of signing the same proposal. This way we do not do any extra computation and can help us when we have a huge unsigned proposal backlog.