routing: export dbMPPayment interface

This will assist external programs attempting to re-use
ChannelRouter code and leverage the ControlTower's FetchPayment
method.

routing/control_tower.go

@@ -9,9 +9,9 @@ import (
 	"github.com/lightningnetwork/lnd/queue"
 )
 
-// dbMPPayment is an interface derived from channeldb.MPPayment that is used by
+// DBMPPayment is an interface derived from channeldb.MPPayment that is used by
 // the payment lifecycle.
-type dbMPPayment interface {
+type DBMPPayment interface {
 	// GetState returns the current state of the payment.
 	GetState() *channeldb.MPPaymentState
 
@@ -76,7 +76,7 @@ type ControlTower interface {
 
 	// FetchPayment fetches the payment corresponding to the given payment
 	// hash.
-	FetchPayment(paymentHash lntypes.Hash) (dbMPPayment, error)
+	FetchPayment(paymentHash lntypes.Hash) (DBMPPayment, error)
 
 	// FailPayment transitions a payment into the Failed state, and records
 	// the ultimate reason the payment failed. Note that this should only
@@ -273,7 +273,7 @@ func (p *controlTower) FailAttempt(paymentHash lntypes.Hash,
 
 // FetchPayment fetches the payment corresponding to the given payment hash.
 func (p *controlTower) FetchPayment(paymentHash lntypes.Hash) (
-	dbMPPayment, error) {
+	DBMPPayment, error) {
 
 	return p.db.FetchPayment(paymentHash)
 }

routing/mock_test.go

@@ -504,7 +504,7 @@ func (m *mockControlTowerOld) FailPayment(phash lntypes.Hash,
 }
 
 func (m *mockControlTowerOld) FetchPayment(phash lntypes.Hash) (
-	dbMPPayment, error) {
+	DBMPPayment, error) {
 
 	m.Lock()
 	defer m.Unlock()
@@ -776,7 +776,7 @@ func (m *mockControlTower) FailPayment(phash lntypes.Hash,
 }
 
 func (m *mockControlTower) FetchPayment(phash lntypes.Hash) (
-	dbMPPayment, error) {
+	DBMPPayment, error) {
 
 	args := m.Called(phash)
 
@@ -814,7 +814,7 @@ type mockMPPayment struct {
 	mock.Mock
 }
 
-var _ dbMPPayment = (*mockMPPayment)(nil)
+var _ DBMPPayment = (*mockMPPayment)(nil)
 
 func (m *mockMPPayment) GetState() *channeldb.MPPaymentState {
 	args := m.Called()

routing/payment_lifecycle.go

@@ -109,7 +109,7 @@ const (
 
 // decideNextStep is used to determine the next step in the payment lifecycle.
 func (p *paymentLifecycle) decideNextStep(
-	payment dbMPPayment) (stateStep, error) {
+	payment DBMPPayment) (stateStep, error) {
 
 	// Check whether we could make new HTLC attempts.
 	allow, err := payment.AllowMoreAttempts()