Support CONCURRENTLY for index builds

[rockwotj]

We have an automated tool to update our DB schema based on migra, and when adding indexes to existing tables we need to add CONCURRENTLY to the create index command. There should be an option for this.

[crimean-celica]

The same. Would be good to have! Thanks.

[maximsmol]

There are many many other things we should be doing concurrently imo + there are specific techniques for avoiding unnecessary locks on top of that. Some of them have huge overhead (up to copying the entire table like what pg_repack does, but having the option to get automatic zero downtime migrations would be a killer feature