switch+router+server: move NextPaymentID to router

This commit moves the responsibility of generating a unique payment ID
from the switch to the router. This will make it easier for the router
to keep track of which HTLCs were successfully forwarded onto the
network, as it can query the switch for existing HTLCs as long as the
paymentIDs are kept.

The router is expected to maintain a map from paymentHash->paymentID,
such that they can be replayed on restart. This also lets the router
check the status of a sent payment after a restart, by querying the
switch for the paymentID in question.

htlcswitch/switch.go

@@ -213,8 +213,6 @@ type Switch struct {
 	pendingPayments map[uint64]*pendingPayment
 	pendingMutex    sync.RWMutex
 
-	paymentSequencer Sequencer
-
 	// control provides verification of sending htlc mesages
 	control ControlTower
 
@@ -293,16 +291,10 @@ func New(cfg Config, currentHeight uint32) (*Switch, error) {
 		return nil, err
 	}
 
-	sequencer, err := NewPersistentSequencer(cfg.DB)
-	if err != nil {
-		return nil, err
-	}
-
 	return &Switch{
 		bestHeight:        currentHeight,
 		cfg:               &cfg,
 		circuits:          circuitMap,
-		paymentSequencer:  sequencer,
 		control:           NewPaymentControl(false, cfg.DB),
 		linkIndex:         make(map[lnwire.ChannelID]ChannelLink),
 		mailOrchestrator:  newMailOrchestrator(),
@@ -354,8 +346,9 @@ func (s *Switch) ProcessContractResolution(msg contractcourt.ResolutionMsg) erro
 }
 
 // SendHTLC is used by other subsystems which aren't belong to htlc switch
-// package in order to send the htlc update.
-func (s *Switch) SendHTLC(firstHop lnwire.ShortChannelID,
+// package in order to send the htlc update. The paymentID used MUST be unique
+// for this HTLC, otherwise the switch might reject it.
+func (s *Switch) SendHTLC(firstHop lnwire.ShortChannelID, paymentID uint64,
 	htlc *lnwire.UpdateAddHTLC,
 	deobfuscator ErrorDecrypter) ([sha256.Size]byte, error) {
 
@@ -376,11 +369,6 @@ func (s *Switch) SendHTLC(firstHop lnwire.ShortChannelID,
 		deobfuscator: deobfuscator,
 	}
 
-	paymentID, err := s.paymentSequencer.NextID()
-	if err != nil {
-		return zeroPreimage, err
-	}
-
 	s.pendingMutex.Lock()
 	s.pendingPayments[paymentID] = payment
 	s.pendingMutex.Unlock()
@@ -407,6 +395,7 @@ func (s *Switch) SendHTLC(firstHop lnwire.ShortChannelID,
 	// Returns channels so that other subsystem might wait/skip the
 	// waiting of handling of payment.
 	var preimage [sha256.Size]byte
+	var err error
 
 	select {
 	case e := <-payment.err: