Add pool support to postgres driver

:swimmer:



This deprecates the `postgresaxomic` driver which depended on the old pg pool functionality that has since [been deprecated](https://github.com/brianc/node-postgres/pull/274). I've also bumped the PG dependancy.

It's not currently possible to run the test suite when `pool = true`. This is also true for the mysql driver. Seems to be due to the way the driver is passed around (`db.driver.db`) and the way queries are executed (`db.query`, `db.run`) in the tests.

I can try fix this in another PR, but it would touch every single test.

cc @johnlouis-swaine-axomic