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/llplugin/llpluginprocessparent.h

@@ -186,7 +186,9 @@ private:
 
 	static bool sUseReadThread;
 	apr_pollfd_t mPollFD;
+	AIAPRPool mPollFDPool;
 	static apr_pollset_t *sPollSet;
+    static AIAPRPool sPollSetPool;
 	static bool sPollsetNeedsRebuild;
 	static LLMutex *sInstancesMutex;
 	static std::list<LLPluginProcessParent*> sInstances;