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/integrated_routing_context_test.go

@@ -75,7 +75,7 @@ func newIntegratedRoutingContext(t *testing.T) *integratedRoutingContext {
 		finalExpiry: 40,
 
 		mcCfg: MissionControlConfig{
-			ProbabilityEstimatorCfg: ProbabilityEstimatorCfg{
+			AprioriConfig: AprioriConfig{
 				PenaltyHalfLife:       30 * time.Minute,
 				AprioriHopProbability: 0.6,
 				AprioriWeight:         0.5,