Add option to run migration CONCURRENTLY for postgres

[tbicr]

For huge table when you create/drop index on live table for postgres, index creation lock table and for live you can just lock all enable connections and go to downtime when index will be created. So for this cases good case to use CONCURRENTLY.

[Ouradze]

Also, when running a transaction concurrently, we may need to change the migration so that the create is run outside a transaction.