Update SmartThreadPool to latest version 2.2.3 with a major and minor change.

SmartThreadPool code comes from http://www.codeproject.com/Articles/7933/Smart-Thread-Pool
This version implements thread abort (via WorkItem.Cancel(true)), threadpool naming, max thread stack, etc. so we no longer need to manually patch those.
However, two changes have been made to stock 2.2.3.
Major change: WorkItem.Cancel(bool abortExecution) in our version does not succeed if the work item was in progress and thread abort was not specified.
This is to match previous behaviour where we handle co-operative termination via another mechanism rather than checking WorkItem.IsCanceled.
Minor change: Did not add STP's StopWatch implementation as this is only used WinCE and Silverlight and causes a build clash with System.Diagnostics.StopWatch
The reason for updating is to see if this improves http://opensimulator.org/mantis/view.php?id=6557 and http://opensimulator.org/mantis/view.php?id=6586