Profile and optimise service responsible for creating pipelines

Description

It appears that the process of creating pipelines / stages is quite slow.

As a test bench, we should use gitlab-ce configuration which consists of about 100 jobs.

Actions

1. We should profile it to see if there is something we can improve,
2. Optimise relevant parts (SQL / repository access) to make it faster for projects like gitlab-ce.

/cc @ayufan @bikebilly