Guarantee that cache is initialized during startup, and only once,
instead of every time a `MetadataHandler` object is instantiated.
Also, improve logic to determine `fixtures` paths.
Introduce an asynchronous Redis instance to be used in async functions.
Also, this change migrates most of the sync cache usage to the new async
cache.
This change applies the guided migration process recommended by
SQLAlchemy [1], up to step 4, to have declarative ORM models that better
support Python typing.
The change was tested by running `alembic check`, which does not find
any schema changes.
Errors reported by `mypy` go down to 170, from the original 223 in the
current `master` commit.
[1] https://docs.sqlalchemy.org/en/20/changelog/whatsnew_20.html#migrating-an-existing-mapping
