Allow AsyncConnectionWrapper to be unwrapped

[xtexx]

AsyncConnectionWrapper::from takes the ownership of AsyncConnection, but there isn't a way to unwrap and take the ownership back.

This can be useful when the connection wrapper is only used for a short time. For example, when used with diesel_migrations, if we can't unwrap the connection back, we will have to open a new connection for migrating, and the connection cannot be re-pooled after using.

Another use case is that, when running unit tests with SQLite in memory database, it is important not to re-open connection as all data will be lost.

[weiznich]

I'm happy to accept a PR for this as long as it comes with documentation + tests.