Categorize log messages into classes to make logs more readable

[prajnoha]

If we use detailed logging, we don't necessarily need to be interested in certain kind of messages and they just make the log uselessly long and tiresome to read.

We should go through all existing log messages and put each one into a category, creating sane categories as needed. Then we can enable only the categories we need.

There's already some preliminary support for categorization in log.c/log_output via class_id arg:

void log_output(int level_id, const char *prefix, int class_id, int errno_id,                                                                                                                                        
                const char *file_name, int line_number, const char *function_name,
                const char *format, ...)

So revisit this part and enhance if needed.