rpcServer: ensure all payIntents are handled before exiting
rpcServer: ensure all payIntents are handled before exiting

This commit fixes a problem that could arise when handling a continuous
stream of payIntents. We would risk the client sending a set of payment
intents and closing the stream, but we wouldn't be sure the sendLoop had
read all messages on the `payChan` before exiting with the nil error
from the `errChan`.

Instead, we close the `payChan` to indicate that no more payments are
going to be received.

LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd,aakselrod/lnd