multi: wrap logger to request shutdown from signal on critical error

This commit adds a shutdown logger which will send a request for
shutdown on critical errors. It uses the signal package to request safe
shutdown of the daemon. Since we init our logs in config validation,
we add a started channel to the signal package to prevent the case where
we have a critical log after the ShutdownLogger has started but before
the daemon has started listening for intercepts. In this case, we just
ignore the shutdown request.

build/logrotator.go

@@ -39,7 +39,10 @@ func NewRotatingLogWriter() *RotatingLogWriter {
 	logWriter := &LogWriter{}
 	backendLog := btclog.NewBackend(logWriter)
 	return &RotatingLogWriter{
-		GenSubLogger:     backendLog.Logger,
+		GenSubLogger: func(tag string) btclog.Logger {
+			logger := backendLog.Logger(tag)
+			return NewShutdownLogger(logger)
+		},
 		logWriter:        logWriter,
 		backendLog:       backendLog,
 		subsystemLoggers: SubLoggers{},