Make it possible to focus debug/info logging to a specific part of MaxScale

This would make the info log a more useful tool for figuring out why something isn't working as expected. Some work on this feature has already been implemented in older releases with the support for service-level and session-level logging as well as the support of syslog facilities for certain types of messages.

An improvement would be to further filter the logging by only doing it for certain parts of the system, for example on a per module basis.

At the session level, the log_info filtering can be done in a more simple manner. With the addition of the session level info logging, the need to enable it has to only be done once on session startup. This allows things such as the username, client IP, database or connection attributes to be used as the filtering criteria.