NMS-6466: Expose poller thread pool stats via JMX

Squashed commit of the following:

commit 9c129df122702261d8d733b729e1ce66d22fa52c
Author: Jeff Gehlbach <jeffg@opennms.org>
Date:   Sun Mar 16 14:50:19 2014 -0500

    Start collecting and graphing the new Pollerd JMX stats

commit 48df9b80070b9c65131c20f4cd615b71fcae451f
Author: Jeff Gehlbach <jeffg@opennms.org>
Date:   Sun Mar 16 12:43:30 2014 -0500

    Add the following attributes to the OpenNMS.Pollerd MBean:

     - ActiveThreads
     - TasksTotal
     - TasksCompleted
     - TaskCompletionRatio