This introduces a pull stat type in addition to the push stat type.
A pull stat takes a method on construction which knows how to update the stat on request.
In this way, special interfaces for pull stat collection are not necessary.
These were neither being returned or in many places reused.
Getting packets from a pool rather than deallocating and reallocating reduces memory churn which in turn reduces garbage collection time and frequency.
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.
