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

lntest/itest/lnd_multi-hop-error-propagation_test.go

@@ -204,6 +204,7 @@ out:
 		FinalCltvDelta: int32(carolPayReq.CltvExpiry),
 		TimeoutSeconds: 60,
 		FeeLimitMsat:   noFeeLimitMsat,
+		MaxParts:       1,
 	}
 	sendAndAssertFailure(
 		t, net.Alice,
@@ -240,6 +241,7 @@ out:
 		FinalCltvDelta: int32(carolPayReq.CltvExpiry),
 		TimeoutSeconds: 60,
 		FeeLimitMsat:   noFeeLimitMsat,
+		MaxParts:       1,
 	}
 	sendAndAssertFailure(
 		t, net.Alice,
@@ -300,6 +302,7 @@ out:
 				PaymentRequest: carolInvoice2.PaymentRequest,
 				TimeoutSeconds: 60,
 				FeeLimitMsat:   noFeeLimitMsat,
+				MaxParts:       1,
 			},
 		)
 
@@ -332,6 +335,7 @@ out:
 		PaymentRequest: carolInvoice3.PaymentRequest,
 		TimeoutSeconds: 60,
 		FeeLimitMsat:   noFeeLimitMsat,
+		MaxParts:       1,
 	}
 	sendAndAssertFailure(
 		t, net.Alice,
@@ -381,6 +385,7 @@ out:
 			PaymentRequest: carolInvoice.PaymentRequest,
 			TimeoutSeconds: 60,
 			FeeLimitMsat:   noFeeLimitMsat,
+			MaxParts:       1,
 		},
 		lnrpc.PaymentFailureReason_FAILURE_REASON_NO_ROUTE,
 	)