Investigate memory consumption for GitLab unicorn processes

We want to reduce GitLab's memory consumption. In order to do that we first need to profile our primary processes and determine where our best opportunities are for improvement. @stanhu is going to perform this analysis and suggested that we approach this on three fronts:

• Unicorn (this issue)
• Sidekiq (#49703 (moved))
• Gitaly (gitaly#1298)

I'm creating these issues so we can track our findings and create related issues for specific fixes.