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.

routing/integrated_routing_context_test.go

@@ -73,6 +73,7 @@ func newIntegratedRoutingContext(t *testing.T) *integratedRoutingContext {
 		PenaltyHalfLife:       30 * time.Minute,
 		AprioriHopProbability: 0.6,
 		AprioriWeight:         0.5,
+		CapacityFraction:      testCapacityFraction,
 	}
 	estimator, err := NewAprioriEstimator(aCfg)
 	require.NoError(t, err)