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.

server.go

@@ -880,6 +880,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 				AprioriHopProbability: aCfg.HopProbability,
 				PenaltyHalfLife:       aCfg.PenaltyHalfLife,
 				AprioriWeight:         aCfg.Weight,
+				CapacityFraction:      aCfg.CapacityFraction,
 			}
 
 			estimator, err = routing.NewAprioriEstimator(