Postgres: Decrease log_min_duration_statement to 500ms

MR: https://dev.gitlab.org/cookbooks/chef-repo/merge_requests/2436

Parent issue gitlab-com/database#145 (moved)

This is the 2nd step of incremental decreasing of log_min_duration_statement.

The considerations proving that the step is safe and will not cause high write I/O: gitlab-com/database#145 (comment 96605155)

The previous issue (setting log_min_duration_statement to 1s): https://gitlab.com/gitlab-com/production/issues/428

Applying this setting does not require a PostgreSQL restart.

Instructions to deploy and validate (same way as it was done previously): https://gitlab.com/gitlab-com/production/issues/428#note_95951367