4lw and jmx should expose the size of the datadir/datalogdir

There are no metrics currently available on the size of the datadir/datalogdir. These grow w/o bound unless the cleanup script is run. It would be good to expose these metrics through jmx/4lw such that monitoring can be done on the size. Would key ppl in on whether cleanup was actually running. In particular this could be monitored/alerted on by third party systems (nagios, ganglia and the like).