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.

rpcserver.go

@@ -7341,7 +7341,7 @@ func (r *rpcServer) DebugLevel(ctx context.Context,
 	if req.Show {
 		return &lnrpc.DebugLevelResponse{
 			SubSystems: strings.Join(
-				r.cfg.LogWriter.SupportedSubsystems(), " ",
+				r.cfg.SubLogMgr.SupportedSubsystems(), " ",
 			),
 		}, nil
 	}
@@ -7350,12 +7350,12 @@ func (r *rpcServer) DebugLevel(ctx context.Context,
 
 	// Otherwise, we'll attempt to set the logging level using the
 	// specified level spec.
-	err := build.ParseAndSetDebugLevels(req.LevelSpec, r.cfg.LogWriter)
+	err := build.ParseAndSetDebugLevels(req.LevelSpec, r.cfg.SubLogMgr)
 	if err != nil {
 		return nil, err
 	}
 
-	subLoggers := r.cfg.LogWriter.SubLoggers()
+	subLoggers := r.cfg.SubLogMgr.SubLoggers()
 	// Sort alphabetically by subsystem name.
 	var tags []string
 	for t := range subLoggers {