routerrpc+routing: adapt payment session for multi shard send

Modifies the payment session to launch additional pathfinding attempts
for lower amounts. If a single shot payment isn't possible, the goal is
to try to complete the payment using multiple htlcs. In previous
commits, the payment lifecycle has been prepared to deal with
partial-amount routes returned from the payment session. It will query
for additional shards if needed.

Additionally a new rpc payment parameter is added that controls the
maximum number of shards that will be used for the payment.

rpcserver.go

@@ -3962,6 +3962,10 @@ func (r *rpcServer) dispatchPaymentIntent(
 			DestCustomRecords: payIntent.destCustomRecords,
 			DestFeatures:      payIntent.destFeatures,
 			PaymentAddr:       payIntent.paymentAddr,
+
+			// Don't enable multi-part payments on the main rpc.
+			// Users need to use routerrpc for that.
+			MaxHtlcs: 1,
 		}
 
 		preImage, route, routerErr = r.server.chanRouter.SendPayment(