routing: rename apriori estimator

* we rename the current probability estimator to be the "apriori"
  probability estimator to distinguish from a different implementation
  later
* the AprioriEstimator is exported to later be able to type switch
* getLocalPairProbability -> LocalPairProbabiltiy (later part of an
  exported interface)
* getPairProbability -> getPairProbabiltiy (later part of an exported
  interface)

lnrpc/routerrpc/router_server.go

@@ -472,7 +472,7 @@ func (s *Server) SetMissionControlConfig(ctx context.Context,
 	error) {
 
 	cfg := &routing.MissionControlConfig{
-		ProbabilityEstimatorCfg: routing.ProbabilityEstimatorCfg{
+		AprioriConfig: routing.AprioriConfig{
 			PenaltyHalfLife: time.Duration(
 				req.Config.HalfLifeSeconds,
 			) * time.Second,