db/DatabasePlugin: pass EventThread's EventLoop to create()

Allows database plugins to use the EventThread, e.g. for CURL
integration.

src/db/DatabaseGlue.hxx

@@ -36,6 +36,7 @@ class Database;
  */
 Database *
 DatabaseGlobalInit(EventLoop &main_event_loop,
+		   EventLoop &io_event_loop,
 		   DatabaseListener &listener,
 		   const ConfigBlock &block);