Cleans-up the schema delta files:
* Removes no-op functions.
* Adds missing type hints to function parameters.
* Fixes any issues with type hints.
This also renames one (very old) schema delta to avoid a conflict
that mypy complains about.
This avoids the overhead of searching through the various
configuration classes by directly referencing the class that
the attributes are in.
It also improves type hints since mypy can now resolve the
types of the configuration variables.
The hope here is that by moving all the schema files into synapse/storage/schema, it gets a bit easier for newcomers to navigate.
It certainly got easier for me to write a helpful README. There's more to do on that front, but I'll follow up with other PRs for that.
