Change default settings for pgbouncer

This makes the default pgbouncer settings more reasonable, reducing the effort required by customers to get things up and running.

These settings are based on the settings for GitLab.com, but slightly lower since most users won't need such a beefy setup.

One thing to take into account:

max_client_conn requires that ulimit -s is at least the same value or greater. It seems Omnibus sets this to 10240 in config/templates/runit/runsvdir-start.erb (line 32), so I think this should be fine.