not-custodial zap beta

[huumn]

The state machine has been tested (absent corner cases) and works reliably on the happy path. This will depend on #1194 and #1195.

Notable things:

• implements an lnproxy-like payment forwarding mechanism using carefully created and observed hold invoices
  • I didn't use lnproxy because it maintains state in-memory and requires manual recovery in the case of unexpected errors
• creates an InvoiceForward table to maintain the state of an attempt/ongoing/completed forward
  • an invoice forward has both an Invoice for the incoming payment and a Withdrawl for the outgoing payment (assuming the incoming payment is received)
• organizes the createInvoice code into its own dir api/createInvoice attempting to standardize the interface for receiving wallets
• extends existing invoice/withdrawal subscription code to have a special case should they have an InvoiceForward associated
  • these special cases in worker/wallet is where all the state transitions happen

TODO:

• walk through all possible state transitions of InvoiceForward and edge cases
  • mindful of reentry errors and idempotentcy
  • mindful of unsuccessful state transitions and/or failed persistence of state transitions
• test (obvs)
• retest all autowithdrawal methods because I refactored the code