[Deprecated] Continue to improve performance of Merge Request Changes API under load into S3 tier

With #225322 (closed), performance of the Merge Request Changes API endpoint was improved significantly by approximately 63% from 7.9s to around 2.9s (can fluctuate between 2.1s to 2.9s):

* Environment:                10k
* Environment Version:        13.9.0-pre `adef80adead`
* Option:                     60s_200rps
* Date:                       2021-02-18
* Run Time:                   1h 18m 27.86s (Start: 04:35:52 UTC, End: 05:54:20 UTC)
* GPT Version:                v2.6.0
❯ Overall Results Score: 98.07%
NAME                                                     | RPS   | RPS RESULT           | TTFB AVG  | TTFB P90             | REQ STATUS     | RESULT
---------------------------------------------------------|-------|----------------------|-----------|----------------------|----------------|-------
api_v4_projects_merge_requests_merge_request_changes     | 200/s | 131.14/s (>16.00/s)  | 1385.11ms | 2078.09ms (<12000ms) | 99.97% (>99%)  | Passed

Screenshot_2021-02-19_at_10.19.30

While this is still in the severity2 performance targets tier that piece of work was significant and for further improvements more separate pieces of work need to be explored so a new issue is being opened to track this.

Of note compared to the previous issue the endpoint is showing a heavier strain on the GitLab Rails nodes (CPU specifically)

Task is to continue this great work and improve performance further into the severity3 tier (<2000ms).

Deprecated

The changes API endpoint has been deprecated and will not be worked on. It'll be removed in API v5. A new diffs API endpoint has been introduced which supports pagination to replace it.

Edited by Patrick Bajao