[features] add pool crate feature

[fakeshadow]

Related discussion: #187

[weiznich]

Looks good, thanks for opening this PR 👍

Just to confirm: You are mostly interested in having access to the PoolableConnection trait for your third party connection type, right?

[fakeshadow]

Looks good, thanks for opening this PR 👍

Just to confirm: You are mostly interested in having access to the PoolableConnection trait for your third party connection type, right?

Thanks for the quick review and merge.
Yes the PoolableConnection trait is essential for both my connection type and my pooling type. With it my connection type can use pooling feature from diesel-async like bb8 and deadpool. And at the same time my pooling type can use connection type from diesel-async as AsyncPgConnection and AsyncMySqlConnection.
Other public types and trait from pooled_connection are good to have for keeping a consistent api setup but they are not must keeping as public.