Make debug logging togglable.

Debug logging adds really a lot of messages to the log file on bigger servers. Currently a lot of servers disable it using a custom log4j2 configuration, but on that way they will not get updates to the default log4j configuration (like the startup time fix recently) automatically also it requires a bit of effort to enable or disable it again. So I think we should add an option for it in the configuration file and make it togglable faster.

If debug logging is enabled in the Spigot configuration then it will override the log4j2 configuration used on the server - if it is disabled it will just do nothing. It would be also possible to enable/disable it at runtime with a command, but I can't really see any use case for that that's why I didn't add that.