Postgres driver not running migrations with the configured schema

[dhui]

Describe the Bug
It looks like the schema is only used for generating the advisory lock id and for dropping migrations.

Steps to Reproduce
See: #262 (comment)

Expected Behavior
Migrations should not need to run SET SCHEMA. Instead, the Postgres db driver should handle that for you

Migrate Version
v4.9.1

Loaded Source Drivers
N/A

Loaded Database Drivers
N/A

Go Version
1.14

Stacktrace
N/A

Additional context
N/A

Potential Solutions

1. I think we can add a method (ensureUsingSchema()) to the postgres in a backwards compatible manner. Not sure if we should run ensureUsingSchema() before every migration since that may break existing migrations, we could run it when the connection opens. That way any exist SET SCHEMA call is honored and subsequent migrations would continue to use the set schema.

Testcases
Any fix should test the following cases:

Where schema specified:	In config	Not in config
In migration
Not in migration

[feketegy]

@dhui Bare in mind that SET SCHEMA in Postgres is fairly new, to cover older versions SET search_path TO my_schema;

[dhui]

Postgres 9.5 is the oldest supported version and it looks like it supports SET SCHEMA.

Another option would be to use SET search_path TO directly.

[feketegy]

SET SCHEMA my_schema is basically a shorthand for SET search_path TO my_schema. Postgres is doing that in the background.