Update public transactions

[annamcallister]

Opening in draft so that I can get some feedback from Cheng ahead of being done.

The missing features I think this needs to be considered done are:

• more validation that updates are valid/safe - particularly around gas pricing
• recalculating gas estimate if transaction data has changed
• Adding sub status updates to record values which have changed

I also need to spend some time thinking about whether there are any updates we want to allow which set a field back to nil or its default.

But I don't think any of these will change the overall structure and approach of the code.

Key points on the approach:

• update is a best effort- return 200 once the changes are persisted into our DB, no promise of what will make it onto the chain
• processing on previous values stops as soon as the changes are persisted, except to persist any work already done
• All submissions are recorded, the comparing hashes with the receipt tells us which submission made it onto the chain