logger: don't call Getpid() inside the Format function

[SandyXSD]

Format is called with the logger mutex locked.

func (entry *Entry) write() {
	entry.Logger.mu.Lock()
	defer entry.Logger.mu.Unlock()
	serialized, err := entry.Logger.Formatter.Format(entry)
	if err != nil {
		fmt.Fprintf(os.Stderr, "Failed to obtain reader, %v\n", err)
		return
	}
	if _, err := entry.Logger.Out.Write(serialized); err != nil {
		fmt.Fprintf(os.Stderr, "Failed to write to log, %v\n", err)
	}
}