MINOR: lua: Add flags to configure logging behaviour

MINOR: lua: Add flags to configure logging behaviour

Until now, messages printed from LUA log functions were sent both to
the any logger configured for the current proxy, and additionally to
stderr (in most cases)

This introduces two flags to configure LUA log handling:
- tune.lua.log.loggers to use standard loggers or not
- tune.lua.log.stderr to use stderr, or not, or only conditionally

This addresses github feature request #2316

This can be backported to 2.8 as it doesn't change previous behaviour.