Remove dependency on marshal and many other async changes

This makes a number of changes:
  * Remove the dependency on marshal by using string.dump and loadstring.
  * Use lua_tolstring rather than having Lua functions pass string lengths to C++.
  * Move lua_api/l_async_events.* to cpp_api/s_async.*, where it belongs.
  * Make AsyncWorkerThread a child of ScriptApiBase, this removes some duplicate functionality.
  * Don't wait for async threads to shut down.  (Is this safe?  Might result in corruption if the thread is writing to a file.)
  * Pop more unused items from the stack
  * Code style fixes
  * Other misc changes

src/httpfetch.cpp

@@ -45,7 +45,7 @@ HTTPFetchRequest::HTTPFetchRequest()
 	caller = HTTPFETCH_DISCARD;
 	request_id = 0;
 	timeout = g_settings->getS32("curl_timeout");
-	connect_timeout = timeout * 5;
+	connect_timeout = timeout;
 	
 	useragent = std::string("Minetest/") + minetest_version_hash + " (" + porting::get_sysinfo() + ")";
 }