Refactor out connection pooling parts of adaptive executor

The usecase is that then we can use it for COPY too (if configured), so we can solve issues like https://github.com/citusdata/citus/issues/710 and https://github.com/citusdata/citus/issues/1047.

I am not sure how would it look like, if the suggestion makes sense, @serprex or I can start on creating an API proposal for this.

@marcocitus any reservations on this?