postgres -> postgres_backend

[aumetra]

Analogue to #707 but for the postgres feature in Diesel.
Makes stuff easier when exclusively using diesel_async.

[Logarithmus]

@Tony-Samuels @paupino this commit somehow doesn't exist in 1.38.0 version of rust_decimal from crates.io.
rust_decimal still pulls pq-sys and it expects libpq-dev to be installed. I use diesel-async in my project with native Rust Postgres driver, so it pollutes my dependency tree.

https://docs.rs/crate/rust_decimal/1.38.0/features

[Tony-Samuels]

@Logarithmus , this is expected as the master branch is currently preparing for a (distant) v2. Changes aimed at the current v1 release need to be merged into the v1 branch.

Apologies for not pointing that out at the time I merged this, I've tried to do so consistently but evidently forgot to warn you about it this time.

[Tony-Samuels]

If you raise a similar MR against v1 I'll happily approve it and it will be made available in the next release.