Allow specifying a finer per-topic compile-time log level

[zah]

Experience has demonstrated that the current log level settings are rather inadequate. A developer is quite often inclined to enable the tracing logging level only for a particular topic of interest (e.g. the modules they are currently working on).

The chronicles_log_level option should support specifying a list of topics with different log level in a way similar to the chronicles_enabled_topics option. The enabled topics option is not sufficient by itself, because quite often the user wants all topics to be enabled, but certain topics to have a different log level.

Design suggestions are very much welcome here - with the right syntax, perhaps a single option can be used to take the role of the currently available multiple options.