Slot reservations implementation

[emizzle]

Defined in the slot reservations codex-research PR codex-storage/codex-research#190

Subtasks:

• Contracts
  - Allow 3(?) slots to be reserved, reserveSlot (returning bool if successful), return true for now
  stub for canReserveSlot(address, SlotId), return true for now, call from reserveSlot
  Client
  - Wire up reserveSlot and canReserveSlot contract calls, but don't call them
• Contracts
  - Add disperal parameter #115 (add to StorageRequest struct only)
  Client
  - Support dispersal parameter (API, API spec, StorageRequest object, etc)
• Client
  - Add SaleReserving state -- SalePreparing -> SaleReserving
  - Refactor slotqueue's seen flags (seen > seenNoAvailability, add seenNotEligible, pause on seenNoAvailability, but don't pause on seenNotEligible)?
• Contracts
  - Third reservation emits SlotFullyReserved event
  Client
  - Slot queue listens for SlotFullyReserved and removes slot from queue
• Contracts
  - Add calculation for expanding window (Expanding window mechanism implementation #116) (use blockhash at time of createStorageRequest for random base SlotID?
• Contracts
  - Disallow fillSlot if slot was not reserved (return false)

[emizzle]

No longer using this task as a parent task, using the epic codex-storage/codex-pm#45 instead.