QueryModel design - Default for #[diesel(check_for_backend()]

[weiznich]

For the new QueryModel derive introduced in #4600 it's an open question if we want to default to generating the relevant code for #[diesel(check_for_backend()] for all backends enabled at compilation time.

From my point of view the advantage of doing that automatically are much better error messages. The disadvantage is that this likely won't work in code-bases that support multiple backends or use a non-default backend (e.g. third party backends or #[derive(MultiConnection)]. For the later case manual annotations would be still required.