routing+lnrpc: make capacity factor configurable

We make the capacity factor configurable via an lnd.conf routerrpc
apriori parameter. The capacity factor trades off increased success
probability with a reduced set of channel candidates, which may lead to
increased fees. To let users choose whether the factor is active or not,
we add a config setting where a capacity fraction of 1.0 disables the
factor. We limit the capacity fraction to values between 0.75 and 1.0.
Lower values may discard too many channels.

lnrpc/routerrpc/routing_config.go

@@ -63,6 +63,9 @@ type AprioriConfig struct {
 	// PenaltyHalfLife defines after how much time a penalized node or
 	// channel is back at 50% probability.
 	PenaltyHalfLife time.Duration `long:"penaltyhalflife" description:"Defines the duration after which a penalized node or channel is back at 50% probability"`
+
+	// CapacityFraction defines the fraction of channels' capacities that is considered liquid.
+	CapacityFraction float64 `long:"capacityfraction" description:"Defines the fraction of channels' capacities that is considered liquid. Valid values are in [0.75, 1]."`
 }
 
 // BimodalConfig defines parameters for the bimodal probability.