Added a thread-safe and robust wrapper for APR pools.

See http://redmine.imprudenceviewer.org/issues/590
and https://jira.secondlife.com/browse/SNOW-596

indra/llcommon/llworkerthread.h

@@ -200,7 +200,7 @@ protected:
 	U32 mRequestPriority; // last priority set
 
 private:
-	LLMutex mMutex;
+	LLMutexRootPool mMutex;		// Use LLMutexRootPool since this object is created and destructed by multiple threads.
 	LLAtomicU32 mWorkFlags;
 };