Improve default log4j.properties

To enable better performance profiling (and for general usefulness), we should log the thread name in the log4j output, as such:

kernel/kernel-common/src/main/config/log4j.properties:

log4j.appender.Sakai.layout.ConversionPattern=%d

%t %5p %F:%L %c

:%L - %m%n

However, this pattern should be generally reviewed, e.g. to include a proper date format (with yyyy-mm-dd as well as time), and also in light of:

http://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/PatternLayout.html

which warns that including the caller info and location (%F %L) is slow and potentially a performance issue, viz.

"WARNING Generating caller location information is extremely slow. It's use should be avoided unless execution speed is not an issue."