RPC: Implement payment codes
The payment code RPCs comprise the last set of RPCs planned for the new renter-host protocol. Briefly, there will be RPCs for creating payment codes and a more general RPC called "block for payment" that instructs the host to wait for an out-of-band payment. The effect of payment codes on existing RPCs is not clear; ideally, payment codes would remove the need for a round-trip or two (since no signatures need to be exchanged), but this would complicate the existing RPC logic. More study needed, so no concrete deadlines yet.