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/llapp.cpp

@@ -123,13 +123,8 @@ void LLApp::commonCtor()
 		mOptions.append(sd);
 	}
 
-	// Make sure we clean up APR when we exit
-	// Don't need to do this if we're cleaning up APR in the destructor
-	//atexit(ll_cleanup_apr);
-
 	// Set the application to this instance.
 	sApplication = this;
-
 }
 
 LLApp::LLApp(LLErrorThread *error_thread) :