routing: split launchShard into registerAttempt and sendAttempt

This commit removes the method `launchShard` and splits its original
functionality into two steps - first create the attempt, second send the
attempt. This enables us to have finer control over "which error is
returned from which system and how to handle it".

channeldb/mp_payment.go

@@ -264,6 +264,7 @@ func (m *MPPayment) InFlightHTLCs() []HTLCAttempt {
 
 // GetAttempt returns the specified htlc attempt on the payment.
 func (m *MPPayment) GetAttempt(id uint64) (*HTLCAttempt, error) {
+	// TODO(yy): iteration can be slow, make it into a tree or use BS.
 	for _, htlc := range m.HTLCs {
 		htlc := htlc
 		if htlc.AttemptID == id {