Testgres

And revert cdc933ff4af.

Small fix: `PostgresNode.safe_psql()` now raises error on wrong input (issue 52).

Small fix: make `NodeBackup` aware of inheritors of `PostgresNode`.

* Add `suppress` option to `PostgresNode.poll_query_until()` (PR 51);
* Add `format` option to `PostgresNode.dump()` (PR 32);
* Add `promote()` method to `PostgresNode` (PR 47);
* Add `version` property to `PostgresNode` (PR 49);
* Add support for logical replication (PR 42);

* Add `wait` option to `PostgresNode.start()` (PR 45);
* `PostgresNode.reload()` now returns `self`;
* Add support for replication slots (PR 41);