Routing checks handlers in each request

[sk-]

The logic in routing.py checks whether the handler is async in each request.

https://github.com/encode/starlette/blob/554f368809e0d891a699667faf0cfbb20057eeb2/starlette/routing.py#L71

This does not seem to be required and if we do the check only once at the beginning of request_response, we achieve some performance gains as seen in https://github.com/encode/starlette/pull/2536/files

[Kludex]

• Worked on refactor: check endpoint handler is async only once #2536. Thanks!