Long-form Renter contracts

This is the first major step towards the new Renter architecture, in which the Renter forms long-duration, large-filesize contracts and uses them to store multiple files. This should greatly improve the scalability issues observed with the current scheme (in which a separate contract is made for each file).

Though the change sounds simple in theory, it has far-reaching implications:

• Contracts can no longer be exactly sized
  • The user needs to be able to set an "allowance" that governs how much to spend on contracts
    • Changing the allowance should result in new contracts, but only in certain cases
  • We must gracefully handle situations where the user attempts to upload more data than their allowance can support
• We must be stricter about who we form contracts with, since the stakes are higher
• Contracts must be fully modifiable, supporting append, edit, and delete operations
  • We must store the Merkle tree of each contract; otherwise we'd have to re-hash the data of every file covered by a contract in order to revise
    • To keep Merkle trees small, we must store only the "4 MB hashes"
    • All files must use a 4 MB piece size
  • New protocols must be created for contract negotiation

Here's a high-level breakdown of what I've completed thus far: (note: not all of those marked "complete" are covered in this PR! Sorry for the confusion.)

• move hostdb code to new "contractor" package
• remove concurrency, expiration, and offline checking from renter
• modify contractor to use long-form contracts
  • simplest possible; uses hard-coded values
• move HostPool to Renter
• implement set-once allowance
• add allowance API endpoints
• add metrics for how much allowance has been used
• implement auto-renewing
  • renew unchanged contracts
  • renew changed contracts (with new allowance)
  • form new contracts if renewing fails
• implement modifiable allowance
  • Update on next renew cycle
  • Update immediately
• design and implement new negotiation protocols
• design and implement new .sia format
• achieve satisfactory test coverage
• restore concurrency and expiration
  • restore multi-piece concurrency
  • restore multi-chunk concurrency
• delete file data after expiration
  • (partial; protocol not finished)
• implement data migration (for when too much redundancy is lost)
  • check for hosts going offline
  • check for hosts holding data hostage
  • check for hosts becoming too expensive
  • Note: contractor can't recover file data, only renter can
• add persistence to hostdb
• contractor needs to scan blockchain to confirm that contract txns and revision txns are submitted
  • needs to submit them itself if not submitted by host
  • can't make use of file contracts until they are seen in the blockchain

Unanswered questions:

• Should contractor.Contracts() return all of the contracts, or just the most recent contract formed with each host? If not the latter, how do we avoid revising outdated contracts?
• What do we do if we can't form contracts with enough hosts?