BaseHTTPMiddleware based Middleware Breaks Background Tasks

[cancan101]

The current middleware subclasses BaseHTTPMiddleware which per Kludex/starlette#919 and fastapi/fastapi#2086 will cause all background tasks to run before the response is sent. Better would be to not subclass BaseHTTPMiddleware.

Instead the class would look like:

class DBSessionMiddleware:
    def __init__(self, app....)
....

    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
        with db():
            await self.app(scope, receive, send)