routing: if MaxShardAmt is set, then use that as a ceiling for our splits

In this commit, we thread through the necessary state to allow users to
set a max shard amount. If this value is set, then this'll effectively
serve as a ceiling for all our split attempts. If we need to split,
we'll first try to use `paymentAmt/2`, if that's bigger than
`MaxShardAmt, then we'll use the latter instead.

Ideally in the future we have a dynamic way to automatically set both
the `MaxShardAmt` as well as `MaxParts` for users. Until then exposing
these two new fields will allow us to experiment with setting them
automatically using the RPC interface, and also give users a bit more
control over how we attempt to route payments, akin to coin control for
on-chain payments.

Fixes #4730

routing/integrated_routing_context_test.go

@@ -28,6 +28,7 @@ type integratedRoutingContext struct {
 	target *mockNode
 
 	amt         lnwire.MilliSatoshi
+	maxShardAmt *lnwire.MilliSatoshi
 	finalExpiry int32
 
 	mcCfg          MissionControlConfig
@@ -151,6 +152,10 @@ func (c *integratedRoutingContext) testPayment(maxParts uint32,
 		MaxParts:       maxParts,
 	}
 
+	if c.maxShardAmt != nil {
+		payment.MaxShardAmt = c.maxShardAmt
+	}
+
 	session, err := newPaymentSession(
 		&payment, getBandwidthHints,
 		func() (routingGraph, func(), error) {