htlcswitch: introducing interceptable switch.

In this commit we implement a wrapper arround the switch, called
InterceptableSwitch. This kind of wrapper behaves like a proxy which
intercepts forwarded packets and allows an external interceptor to
signal if it is interested to hold this forward and resolve it
manually later or let the switch execute its default behavior.
This infrastructure allows the RPC layer to expose interceptor
registration API to the user and by that enable the implementation
of custom routing behavior.

server.go

@@ -207,6 +207,8 @@ type server struct {
 
 	htlcSwitch *htlcswitch.Switch
 
+	interceptableSwitch *htlcswitch.InterceptableSwitch
+
 	invoices *invoices.InvoiceRegistry
 
 	channelNotifier *channelnotifier.ChannelNotifier
@@ -515,6 +517,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr, chanDB *channeldb.DB,
 	if err != nil {
 		return nil, err
 	}
+	s.interceptableSwitch = htlcswitch.NewInterceptableSwitch(s.htlcSwitch)
 
 	chanStatusMgrCfg := &netann.ChanStatusConfig{
 		ChanStatusSampleInterval: cfg.ChanStatusSampleInterval,