pkg/log: improve performance for disabled log levels (scionproto#4304)

Check if the log level is enabled before converting the log data
with a call to convertCtx, to reduce the cost of the log call for
disabled log levels.

The following benchmark tests shows an improvement of the execution time
from 135ns to 30ns with this change.

```go
func BenchmarkLog(b *testing.B) {
	err := log.Setup(log.Config{Console: log.ConsoleConfig{Level: "info"}})
	assert.NoError(b, err)
	for i := 0; i < b.N; i++ {
		log.Debug("hello world", "i", i)
	}
}
```

pkg/log/wrappers.go

@@ -22,17 +22,27 @@ import (
 
 // Debug logs at debug level.
 func Debug(msg string, ctx ...interface{}) {
-	zap.L().Debug(msg, convertCtx(ctx)...)
+	if enabled(DebugLevel) {
+		zap.L().Debug(msg, convertCtx(ctx)...)
+	}
 }
 
 // Info logs at info level.
 func Info(msg string, ctx ...interface{}) {
-	zap.L().Info(msg, convertCtx(ctx)...)
+	if enabled(InfoLevel) {
+		zap.L().Info(msg, convertCtx(ctx)...)
+	}
 }
 
 // Error logs at error level.
 func Error(msg string, ctx ...interface{}) {
-	zap.L().Error(msg, convertCtx(ctx)...)
+	if enabled(ErrorLevel) {
+		zap.L().Error(msg, convertCtx(ctx)...)
+	}
+}
+
+func enabled(lvl Level) bool {
+	return zap.L().Core().Enabled(zapcore.Level(lvl))
 }
 
 // WithOptions returns the logger with the options applied.
@@ -72,15 +82,21 @@ func (l *logger) New(ctx ...interface{}) Logger {
 }
 
 func (l *logger) Debug(msg string, ctx ...interface{}) {
-	l.logger.Debug(msg, convertCtx(ctx)...)
+	if l.Enabled(DebugLevel) {
+		l.logger.Debug(msg, convertCtx(ctx)...)
+	}
 }
 
 func (l *logger) Info(msg string, ctx ...interface{}) {
-	l.logger.Info(msg, convertCtx(ctx)...)
+	if l.Enabled(InfoLevel) {
+		l.logger.Info(msg, convertCtx(ctx)...)
+	}
 }
 
 func (l *logger) Error(msg string, ctx ...interface{}) {
-	l.logger.Error(msg, convertCtx(ctx)...)
+	if l.Enabled(ErrorLevel) {
+		l.logger.Error(msg, convertCtx(ctx)...)
+	}
 }
 
 func (l *logger) Enabled(lvl Level) bool {