Make model operations wait on `Model.init()`?

Re: #8786 , other issues.

Testing transactions, `unique` constraints, change streams, etc. is confusing because you need to call `Model.init()` under the hood to wait for indexes to build and for Mongoose to create the collection (if you set the `autoCreate` option). It would be great if we could make Mongoose automatically create collections for models and block `startSession()` and `watch()` until the collection is created.

Also see #8814

Start by reading the discussion in #8786 and #8814, then trace the existing Model.init(), startSession(), and watch() entry points. Define how automatic collection creation and waiting should interact with indexes, transactions, unique constraints, and change streams; done means those operations no longer require callers to invoke Model.init() first.

Written by the indexing model from the issue text.