Add primary key to audit enqueued table

[hlascelles]

In some circumstances it is useful or even necessary for tables to have a primary key, eg Blue / Green deployment.

The que_scheduler_audit_enqueued does not have a primary key, so this PR adds one. This will require a migration so a new major version of que-scheduler is required. Note this may be a DB intensive operation that can take some time if you have been running que-scheduler for a while. Unless you are running the "cleanup job" (QueSchedulerAuditClearDownJob), you will have 1 row per job scheduled enqueued since the audit started.

Use a migration like so:

class CreateQueSchedulerSchema < ActiveRecord::Migration[6.0]
  def change
    Que::Scheduler::Migrations.migrate!(version: 8)
  end
end

The scheduler will pause until the migration to version 8 is completed, as the table will be locked. It may also mean that one que worker will be paused waiting to access that table if it takes so long that the "scheduler run" happens ("on the minute"). This may be fine if you have only tens or hundreds of thousands of rows. If you have millions, and don't wish to have any scheduler pause, it may be best to run a clear down with QueSchedulerAuditClearDownJob first (and maybe on an ongoing basis). See the README.md for more details here.