log: swap logrus functions with their equivalent on default logger

[`logrus.SetLevel()`][1], [`logrus.GetLevel()`][2] and [`logrus.SetFormatter()`][3]
are all convenience functions to configure logrus' standardlogger, which is the
logger to which we hold a reference in the Entry configured on [`log.L`][4].

This patch:

- swaps calls to `logrus.SetLevel`, `logrus.GetLevel` and `logrus.SetFormatter`
  for their equivalents on `log.L`. This makes it clearer what `SetLevel` does,
  and makes sure that we set the log-level of the logger / entry we define in
  our package (even if that would be swapped with a different instance).
- removes the use of `logrus.NewEntry` with directly constructing a `Entry`,
  using the local `Entry` alias (anticipating we can swap that type in future).

[1]: dd1b4c2e81/exported.go (L34C1-L37)
[2]: dd1b4c2e81/exported.go (L39-L42)
[3]: dd1b4c2e81/exported.go (L23-L26)
[4]: dd1b4c2e81/exported.go (L9-L16)

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

log/context.go

@@ -45,7 +45,11 @@ import (
 )
 
 // L is an alias for the standard logger.
-var L = logrus.NewEntry(logrus.StandardLogger())
+var L = &Entry{
+	Logger: logrus.StandardLogger(),
+	// Default is three fields plus a little extra room.
+	Data: make(Fields, 6),
+}
 
 type loggerKey struct{}
 
@@ -116,13 +120,13 @@ func SetLevel(level string) error {
 		return err
 	}
 
-	logrus.SetLevel(lvl)
+	L.Logger.SetLevel(lvl)
 	return nil
 }
 
 // GetLevel returns the current log level.
 func GetLevel() Level {
-	return logrus.GetLevel()
+	return L.Logger.GetLevel()
 }
 
 // OutputFormat specifies a log output format.
@@ -141,13 +145,13 @@ const (
 func SetFormat(format OutputFormat) error {
 	switch format {
 	case TextFormat:
-		logrus.SetFormatter(&logrus.TextFormatter{
+		L.Logger.SetFormatter(&logrus.TextFormatter{
 			TimestampFormat: RFC3339NanoFixed,
 			FullTimestamp:   true,
 		})
 		return nil
 	case JSONFormat:
-		logrus.SetFormatter(&logrus.JSONFormatter{
+		L.Logger.SetFormatter(&logrus.JSONFormatter{
 			TimestampFormat: RFC3339NanoFixed,
 		})
 		return nil