Raise the backlog_socket value

Raise default on Omnibus from 64 to the Unicorn default value of 1024 to increase performance.

Add a note to the config file that they can lower it if they use a load balancer.

http://unicorn.bogomips.org/Unicorn/Configurator.html#method-i-listen https://dev.gitlab.org/gitlab/gitlabhq/blob/master/config/unicorn.rb.example#L33