Event loop is acquired too early

[jonafato]

When the AsyncIOMotorClient is initialized in init_app, asyncio.get_event_loop is called, and a reference to the event loop is stored. This can cause problems when a loop other than the default is used (e.g. uvloop or the loop created by Henson's CLI's reloader).

To address this, the creation of the client should be deferred. The easiest approach here is to use the app.startup hook to create the client when the application is starting rather than during the extension's initialization. (Are there other / better options here, as well?)