This commit removes the context as a param needed when calling methods
of HarnessNode. This change moves the context management inside
HarnessNode, aside from saving us a few lines, it makes the context
creation/timeout less error-prone.
This permits an AMP invoice to be "pseudo-reusable", where the invoice
paramters can be used multiple times so long as a new payment address is
supplied. This prevents additional round trips between payer and payee
to obtain a new invoice, even though the payments/invoices won't be
logically associated via the RPC interface like they would when the full
reusable invoices are deployed.
