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/llmath/llvolumemgr.cpp

@@ -55,7 +55,7 @@ LLVolumeMgr::LLVolumeMgr()
 {
 	// the LLMutex magic interferes with easy unit testing,
 	// so you now must manually call useMutex() to use it
-	//mDataMutex = new LLMutex(gAPRPoolp);
+	//mDataMutex = new LLMutex;
 }
 
 LLVolumeMgr::~LLVolumeMgr()
@@ -222,7 +222,7 @@ void LLVolumeMgr::useMutex()
 { 
 	if (!mDataMutex)
 	{
-		mDataMutex = new LLMutex(gAPRPoolp);
+		mDataMutex = new LLMutex;
 	}
 }