Slot reservations

[markspanbroek]

Describes a mechanism that allows hosts to reserve a slot by depositing collateral early. The main advantage of this is that it eliminates duplicated effort in the network; instead of multiple hosts competing to provide a proof of storage, only a single host will download the content and calculate a proof of storage.

This mechanism has been discussed in the Codex team on various occasions. This is my attempt at capturing it in a design.

I'm not entirely convinced that we should adopt this design, however. My main doubts are around the added complexity in the smart contract (suggestions to simplify the design are very welcome!), and I'm not entirely convinced that the dispersal mechanism cannot be utilized to provide the same functionality. Thoughts?

Closes #158.