Enable non-transactional migrations

For DB safety reasons, sometimes it is necessary to add an index concurrently. With the CONCURRENTLY option:

PostgreSQL will build the index without taking any locks that prevent concurrent inserts, updates, or deletes on the table; whereas a standard index build locks out writes (but not reads) on the table until it's done.

But this parameter cannot be used in migrations because Hasura runs the migration in a transaction when applying. But concurrent index creation cannot be wrapped in a transaction block:

Another difference is that a regular CREATE INDEX command can be performed within a transaction block, but CREATE INDEX CONCURRENTLY cannot.

There don't seem to be any workarounds or escape hatches in this case, beyond not tracking the migration at all and running directly on the db instance.

Some solutions might be to include an option in the apply api, or encoding not wrapping a transaction in the yml for the migration file itself.

Original request in Discord: https://discordapp.com/channels/407792526867693568/535727661167673364/626175223724769295

Postgres documentation: https://www.postgresql.org/docs/9.1/sql-createindex.html