Use thread-safe version of .NET Random as the SDK class is not thread-safe.

As per http://msdn.microsoft.com/en-us/library/system.random%28v=vs.100%29.aspx, the .NET Random class is not thread-safe.
If called by multiple threads at once, methods may return 0.
Except for llRand(), other OpenSimulator code did not lock before calling a shared Random instance.
This commit adds a ThreadSafeRandom class that extends Random but does internal locking so that it is thread-safe.
This change is invisible to existing callers and the explicit locking in the llFrand() implementation is now redundant.