Improve asynchronous pipeline processing
## What does this MR do?
This MR improves asynchronous processing of pipeline.
## Why was this MR needed?
It eliminates some race conditions and improves performance.
## Does this MR meet the acceptance criteria?
- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
- [x] Added for this feature/bug
- [x] All builds are passing
## What are the relevant issue / merge request numbers?
Related merge request: !6410
Extracted from !6411
See merge request !6650
0bbeff3d5e