Update various PostgreSQL default settings
This changes:
- work_mem from 8MB to 16MB
- statement_timeout from 0 (= no timeout) to 60 seconds
- random_page_cost from 4.0 to 2.0
- max_locks_per_transaction from 64 to 128
These settings are based on what we use for GitLab.com.
Originally requested in https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1500#note_33169528.