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.

routing/router.go

@@ -1624,6 +1624,10 @@ type LightningPayment struct {
 	// understand this new onion payload format, then the payment will
 	// fail.
 	DestCustomRecords record.CustomSet
+
+	// MaxHtlcs is the maximum number of partial payments that may be use to
+	// complete the full amount.
+	MaxHtlcs uint32
 }
 
 // SendPayment attempts to send a payment as described within the passed