improve(Dataworker): Remove 0-value refunds and 0-value empty-message slow fills

[nicholaspai]

implements optimizations to UMIP-179

The goal here is protecting dataworker from performing extra computations where no on-chain state changes can happen, in the case of 0-value refunds and 0-value empty-message slow fills.

[pxrl]

+1. How do we synchronise this change on chain? I also have UMAprotocol/UMIPs#602 open - I got stalled on it because it also requires synchronisation - i.e. we need to identify when the transition would occur. We could do this by mainnet block number, or some change to the config store (bumping the VERSION field?).

[nicholaspai]

+1. How do we synchronise this change on chain? I also have UMAprotocol/UMIPs#602 open - I got stalled on it because it also requires synchronisation - i.e. we need to identify when the transition would occur. We could do this by mainnet block number, or some change to the config store (bumping the VERSION field?).

I don't think we need any special logic to be honest, we likely don't need to run the dataworker code on any previous bundles anymore (I'll check no stuck leaves) and we can time this with a bundle with no 0-value refunds/0-value slow fills.

This way we can avoid the version bump

[mrice32]

LGTM!