Claim slot before downloading

[markspanbroek]

Investigate a change to the marketplace design whereby a slot can be claimed by a host before it starts downloading data.

Rationale:

In the current design of the marketplace, a slot can be claimed by a host by offering both collateral and proof of storage. In other words, a host would need to download the data before it's allowed to fill a slot.

This can be problematic. Because hosts are racing to fill a slot, by all downloading the data, they could overwhelm the capacity of the client that is offering the data to the network. Bandwidth incentives and dispersal could alleviate this, but this is far from certain.

This is why it's useful to investigate an alternative: allow a host to claim a slot before it starts downloading the data. A host can claim a slot by providing collateral. This changes the dynamics quite a bit; hosts are now racing to put down collateral, and it's not immediately clear what should happen when a contract fails. The incentives need to be carefully changed to avoid unwanted behavior.