Add 'UseMigrationLock' flag to acquire exclusive lock while performing migrations for Postgres

[zapling]

First I'd like to thank you @amacneil for all the hard work, been using this project for years now and it's the best tool for the job.

We currently have a client who are performing migrations from within their code using the dbmate library to do so. They have multiple components that are run as their own deployment in kubernetes, meaning each component is peforming a migration. When we do new deployments of this application, each component starts up and migrations are attempted at the same time, resulting in race conditions.

We have resolved this by adding a new DriverMigrationLock interface, that the Postgres driver implements. It uses an postgres advisory lock within a database transaction to prevent any other dbmate instance from migrating while the lock is active.

The use of the exclusive lock is currently opt-in by using the UseMigrationLock field on the DB struct.

I feel like this might be something more people would like to have, who might be in a similar position.

I have added a few tests for the Postgres driver, I can add more, but unsure what exactly what we should test? It might be nice to have one test for the DB struct, if so, could you point me in the right direction?