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)

routing/router_test.go

@@ -120,7 +120,7 @@ func createTestCtxFromGraphInstanceAssumeValid(t *testing.T,
 	}
 
 	mcConfig := &MissionControlConfig{
-		ProbabilityEstimatorCfg: ProbabilityEstimatorCfg{
+		AprioriConfig: AprioriConfig{
 			PenaltyHalfLife:       time.Hour,
 			AprioriHopProbability: 0.9,
 			AprioriWeight:         0.5,