Implement Keeper

[sadhbh-c0d3]

1. Keeper is an off-chain service:

• Keeper recieves orders from user
• Keeper negotiates price with Vendor
• Keeper submits order together with prices onchan
• Keeper batches orders (periodically)
• Keeper pushes next batch (periodically)

2. We can have multiple Keepers.

• Keeper does not store any state, and only uses state onchain.
• We could have multiple Vendors (this implies multiple Markets too), then
• Order submission (public) routine must take: order, market data (for order), vendor ID

3. The market data submitted with order does not need to be all assets, and it can be optimised so that only assets for which market data is stale (last time updated older than X).

4. There can be multiple Keepers.

• Keeper spends their own gas on all operations.
• Keeper can be refunded from fees.
• Anyone can run Keeper.