Disable PostgreSQL fsync for faster CI

Turning off durability settings in PostgreSQL in CI appears to improve specs by 16%.

Using the API data, the 50 rspec-pg success test runs between master and this branch:

• https://gitlab.com/api/v4/projects/13083/pipelines/56541883/jobs?per_page=100
• https://gitlab.com/api/v4/projects/13083/pipelines/56462614/jobs?per_page=100

Total before	Total after	Delta	% Speedup
70836 s (19.7 hours)	59652 s (16.6 hours)	11184 s (3.1 hours)	16

require 'json'

def select_pg_jobs(filename)
  jobs = JSON.parse(File.read(filename))
  jobs.select { |x| x['name'].start_with?('rspec-pg ') && x['status'] == 'success' }
end

master = select_pg_jobs('master.json')
fsync = select_pg_jobs('no-fsync.json')

puts master.map { |x| x['duration'] }.sum
puts fsync.map { |x| x['duration'] }.sum

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/60435