Added shutdown hook to close all open RrdDb files held in the pool when the JVM is about to exit. Added method to close() the pool manualy (it's not harmful to close the file more than once).
Added shutdown hook to close all open RrdDb files held in the pool when the JVM is about to exit. Added method to close() the pool manualy (it's not harmful to close the file more than once).

OpenNMS/jrobin,OpenNMS/jrobin,roskens/jrobin,roskens/jrobin,roskens/jrobin,OpenNMS/jrobin,OpenNMS/jrobin