Implement WithConnection for pgx driver

[GAlexIHU]

Contents

Implements WithConnection method for pgx driver

Notes for reviewer

As originally mentioned in this Issue calling .Close also closes the underlying pool not just the connection used which might not be desired in some cases.

The changes mirrors the implementation in the other drivers, e.g. https://github.com/golang-migrate/migrate/blob/master/database/postgres/postgres.go#L68

[coveralls]

coverage: 56.361% (+0.04%) from 56.319% when pulling ce0013366ccca8ac41b2c02529dae8b07eff2477 on GAlexIHU:feat/pgx-withconn into 555501f7cdbf52a34bf66d49eb37eb72b22b2a85 on golang-migrate:master.

[chrisgacsal]

Hey @mattes @dhui! Sorry for poking you, but could you please take a look on this PR? Many thanks! 🙇🏻‍♂️

[GAlexIHU]

closing due to inactivity