build: separate sublogger and rotator pipe management

These are two separate concerns. So this commit splits them up and just
passes a LogWriter from the one to the other. This will become cleaner
in an upcoming commit where the Rotator will implement io.Writer and
there will no longer be a need for LogWriter.

lnd.go

@@ -149,7 +149,7 @@ func Main(cfg *Config, lisCfg ListenerCfg, implCfg *ImplementationCfg,
 
 	defer func() {
 		ltndLog.Info("Shutdown complete\n")
-		err := cfg.LogWriter.Close()
+		err := cfg.LogRotator.Close()
 		if err != nil {
 			ltndLog.Errorf("Could not close log rotator: %v", err)
 		}