Block requests before end of plugin initialization

On huge instances, if there are Etherpad crashes, MyPads may take some time be fully initialized, because Etherpad heavily uses database and data flushes before giving hand to plugins. This can lead to data corruption, as MyPads interface is usable before required in-memory indexes.

To prevent this trouble, we should use an in-memory initialization flag. All requests done before the flag has been set to true must return a generic error, for example "MyPads is loading, please wait".