Arvados

There are several things we to do run websockets well in production that aren't documented in the install guide yet:

• The runit script uses ulimit instead of chpst -m.
• Set the ActiveRecord pool size in API server's database.yml.
• Something about PostgreSQL connections, maybe?

Add all of this to the install guide. Since all the numbers here scale in part with the size of the API server node, we'll need to figure out how we want to express that.