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/pathfind_test.go

@@ -81,6 +81,12 @@ var (
 		), lnwire.Features,
 	)
 
+	mppFeatures = lnwire.NewRawFeatureVector(
+		lnwire.TLVOnionPayloadOptional,
+		lnwire.PaymentAddrOptional,
+		lnwire.MPPOptional,
+	)
+
 	unknownRequiredFeatures = lnwire.NewFeatureVector(
 		lnwire.NewRawFeatureVector(100), lnwire.Features,
 	)