LND.pay_invoice() takes constraints

[nullcount]

for the new channel manager strategy, the balance of every channel needs to be managed. This means, we need a way to specify the first outgoing channel when paying an invoice. Would also be nice to include max fee as a stop loss.

For LND.pay_invoice()

• add an optional arg for the chanid of the first outgoing channel
• add an option arg for max_fee in satoshi to pay for this invoice