Update duration at the end of pipeline

## What does this MR do?
Moves duration calculation to be done only once at the end of pipeline processing.
Currently this is done every one build.

## Why was this MR needed?
This is the simplest thing that we can do before properly implementing duration calculation: https://gitlab.com/gitlab-org/gitlab-ce/issues/23523#note_17145614

This is ~Performance improvement that significantly affects: http://performance.gitlab.net/dashboard/db/sidekiq-workers?var-worker=PipelineUpdateWorker%23perform&var-database=Production&from=now-1h&to=now

See merge request !6987