Add OS memory stats to the node status stats

[at055612]

With the use of off heap stores we need better monitoring of the os memory state (not just the JVM memory as currently handled) from stroom. Suggest stroom.node.impl.NodeStatusServiceUtil uses ManagementFactory.getOperatingSystemMXBean() to capture the following

getCommittedVirtualMemorySize()
getFreePhysicalMemorySize()
getFreeSwapSpaceSize()

See https://docs.oracle.com/javase/7/docs/jre/api/management/extension/com/sun/management/OperatingSystemMXBean.html