First round of server optimization

[renatomassaro]

So far I haven't changed a single config of the servers, so they are far from the ideal setup for production env.

By first round I mean that since we do not have any traffic or data, the best we can do for now is guess (based on gut/experience) the best tunable values. Once we do have some data we can proceed to the second round of optimization.

Special stuff that comes to mind: sysctl, user limit, networking stack, ZFS compression/dedup.

Service-specific optimizations (nginx, haproxy, pg) are out of scope for this issue, but a new one should be create (at least to assess how well optimized they are).