adding a pool feature without any specific pool crate enabled

[fakeshadow]

I'm trying to implement my own db driver and pool implement as an extension crate of diesel-async. In the progress I find pooled_connection module can only be enabled with at least one existing pool crate. Resulting extra dependencies I don't need.

I imagine a feature like this would help resolve the dependency issue:
Cargo.toml

[features]
pool = []
bb8 = ["pool", "dep::bb8"]

mod.rs

[cfg(feature = "pool")]
pub mod pooled_connection;

[weiznich]

That sounds reasonable, happy to accept a PR for this.