lnd: set up messaging chans between peer and htlcSwitch

Each active channel now gains its a dedicated htlcManager goroutine
which currently accepts to two golang channels, and a lightning
channel. The “downstream” channel will be used for dispatched multi-hop
payments sent from the htlcSwitch, while the “upstream” channel will be
used once the readHandler de-multiplexes hltc add/timeout/settle
messages.

Each time a new channel is fully created after N confirmations, the
peer’s channelManager registers the new link with the htlcSwitch. Once
the channel is closed either cooperatively or non-cooperatively, then
the link is unregistered.

log.go

@@ -25,6 +25,7 @@ var (
 	srvrLog    = btclog.Disabled
 	ntfnLog    = btclog.Disabled
 	chdbLog    = btclog.Disabled
+	hswcLog    = btclog.Disabled
 )
 
 // subsystemLoggers maps each subsystem identifier to its associated logger.
@@ -37,6 +38,7 @@ var subsystemLoggers = map[string]btclog.Logger{
 	"NTFN": ntfnLog,
 	"CHDB": chdbLog,
 	"FNDG": fndgLog,
+	"HSWC": hswcLog,
 }
 
 // useLogger updates the logger references for subsystemID to logger.  Invalid
@@ -74,6 +76,9 @@ func useLogger(subsystemID string, logger btclog.Logger) {
 
 	case "FNDG":
 		fndgLog = logger
+
+	case "HSWC":
+		hswcLog = logger
 	}
 }