This allows different categories of stats to be shown, with options to list categories or show all stats.
Currently categories are scene and simulator and only a very few stats are currently registered via this mechanism.
This commit also adds percentage stats for packets and blocks reused from the packet pool.
This increments a SlowFrames counter if a frame takes over 120% of maximum time.
This commit also introduces a generic OpenSim.Framework.Monitoring.Stat which is available to any code that wants to register a statistic.
This is more granualar than asking objects to create their own reports.
At some point this will supersede earlier IMonitor and IAlert facilities in MonitoringModule which are only available to scene code.
