routing: add probability estimator interface

We introduce a probability `Estimator` interface which is implemented by
the current apriori probability estimator. A second implementation, the
bimodal probability estimator follows.

routing/probability_estimator.go

@@ -0,0 +1,37 @@
+package routing
+
+import (
+	"time"
+
+	"github.com/btcsuite/btcd/btcutil"
+	"github.com/lightningnetwork/lnd/lnwire"
+	"github.com/lightningnetwork/lnd/routing/route"
+)
+
+// Estimator estimates the probability to reach a node.
+type Estimator interface {
+	// PairProbability estimates the probability of successfully traversing
+	// to toNode based on historical payment outcomes for the from node.
+	// Those outcomes are passed in via the results parameter.
+	PairProbability(now time.Time, results NodeResults,
+		toNode route.Vertex, amt lnwire.MilliSatoshi,
+		capacity btcutil.Amount) float64
+
+	// LocalPairProbability estimates the probability of successfully
+	// traversing our own local channels to toNode.
+	LocalPairProbability(now time.Time, results NodeResults,
+		toNode route.Vertex) float64
+
+	// Config returns the estimator's configuration.
+	Config() estimatorConfig
+
+	// String returns the string representation of the estimator's
+	// configuration.
+	String() string
+}
+
+// estimatorConfig represents a configuration for a probability estimator.
+type estimatorConfig interface {
+	// validate checks that all configuration parameters are sane.
+	validate() error
+}