routing+channeldb: make MPPayment into an interface

This commit turns `MPPayment` into an interface inside `routing`. Having
this interface gives us the benefit to write more granular unit tests
inside payment lifecycle. As seen from the modified unit tests, several
hacky ways of testing the `SendPayment` method is now replaced by a mock
over `MPPayment`.

routing/payment_lifecycle_test.go

@@ -791,12 +791,6 @@ func testPaymentLifecycle(t *testing.T, test paymentLifecycleTestCase,
 	}
 }
 
-func makeActiveAttempt(total, fee int) channeldb.HTLCAttempt {
-	return channeldb.HTLCAttempt{
-		HTLCAttemptInfo: makeAttemptInfo(total, total-fee),
-	}
-}
-
 func makeSettledAttempt(total, fee int,
 	preimage lntypes.Preimage) channeldb.HTLCAttempt {