Implemented flush, added a call to it from the filter speaker's destructor, to ensure no race conditions on accessing the various bits of instance state there and below.

Concurrency/AsyncTaskQueue.hpp

@@ -39,7 +39,7 @@ class AsyncTaskQueue {
 		/*!
 			Blocks the caller until all previously-enqueud functions have completed.
 		*/
-		void synchronise();
+		void flush();
 
 	private:
 		std::unique_ptr<std::thread> thread_;