Minting: make sure we can transition from current flow

[DylanVerstraete]

According to following specs:

The current minting has 2 things which we need to account for. These are fixed periods, and receipts. Receipts will be replaced by elaborate events when the minting happens, which will contain all the data the current receipts have. These events can then easily be extracted from graphql by whoever wants, possibly to build a UI on top.

The fixed periods also make it easy to transition from one system to the other. Once the code is complete, we decide when to transition. This will be at the boundary of a period. We then add checks in the minting code on chain to ignore most events from before this timestamp, only updating the last report received and last reported uptime fields. After the timestamp we handle extrinsics as described. In a future update, the checks can be removed. We then do a final mint off chain while the minting code is running and takes over.

[DylanVerstraete]

Will require work in graphql: threefoldtech/tfchain_graphql#82