Allowing multiple connections can lead to `set role` commands not being run against every connection

[JAForbes]

We've recently observed this, a prior migration used:

export const connection = {
  max: 4
}

And then the subsequent migration created a table but the set role $migration_user command must have run on a different connection because the roles were created with postgres ownership.

Will need to potentially bring back killing all connections between migration files if we allow parallel connections, or at least if the prior migration had max > 1.