This allows streaming large result sets instead of bufferring them.
The change is fairly invasive because the state machine has to be adapted to allow returning a `PgResponse` before the client is allowed to dispatch other requests on the connection.
This is handled in the dispatcher where it expects such `PgResponse`s to provide a signal to release the connection.

NOTE: this doesn't apply the strategy to prepared statements yet. Looking for feedback before moving forward.

This also applies the streaming of rows to the extended query mode.