runtime: Transaction auctioning

[NunoAlexandre]

We want the mining node to prioritize transaction for inclusion based on the associated paid tx fee. Even though we set this priority already, it is unclear and untested how miners prioritize transactions in practice at the moment. We need to check whether the Substrate’s transaction weight system overrules this set priority or not. We don’t care about the runtime cost for now.

• Come up with a plan to test the prioritization. (Not necessary CI but we should have some tools to send a lot of transactions with different priorities)
• Evaluate whether we should remove substrates weight and transaction fee system