Allow to create indexes later on the update process

When updating the modules on big databases the index creation take a long time to complete.

We could have a switch on trytond-admin in order to delay the creation of the indexes. That way the database schema is already ready but the performance gain of using the indexes are not available yet. Another run without the switch will create the indexes and it can be used at a more appropriate time.

Since PostgreSQL allows concurrent index creation it would nice to have this opportunity too.