core: Add a profiling API and implement some profiling points

core: Add a profiling API and implement some profiling points

The API outputs messages in the format accepted by Federico’s
plot-timeline.py (http://people.gnome.org/~federico/news-2006-03.html#timeline-tools)
so graphs can easily be generated by running:

    strace -ttt -f -o /tmp/logfile folks-inspect
    python plot-timeline.py -o graph.png /tmp/logfile

So far, the BackendStore, IndividualAggregator and Backend and PersonaStore
classes for all the backends have been annotated.