New contract protocol (Renter side)

The protocol code is almost ready, but not quite. There is some tricky signature stuff that I wasn't comfortable implementing on my own.

This PR also adds some spending metrics, and persistent subscription.

Not yet implemented:

• If the connection drops during revision, the renter should immediately attempt to reconnect and resend the last action.
• Need to figure out how host pubkeys should be handled by the hostdb. Right now I'm just assuming a PublicKey field in the host settings.
• The contract protocol should be documented extensively somewhere, perhaps in the doc folder. We wrote it down earlier so it shouldn't be hard to convert to a more readable format.