Investigate Postgres CPU usage spikes on 50k

Investigate 2 failures in the latest GPT results from 2020-08-03: api_v4_projects_project and api_v4_projects_merge_requests_merge_request.

* Environment:                50k
* Environment Version:        13.3.0-pre `490f1902e6f`
* Option:                     60s_1000rps
* Date:                       2020-08-03
* Run Time:                   1h 18m 35.34s (Start: 13:05:47 UTC, End: 14:24:23 UTC)
* GPT Version:                v2.0.5
❯ Overall Results Score: 95.64%
NAME                                                     | RPS    | RPS RESULT           | TTFB AVG  | TTFB P90              | REQ STATUS     | RESULT 
---------------------------------------------------------|--------|----------------------|-----------|-----------------------|----------------|--------
api_v4_projects_merge_requests_merge_request             | 1000/s | 888.98/s (>800.00/s) | 977.56ms  | 1159.76ms (<500ms)    | 100.00% (>95%) | FAILED²
api_v4_projects_project                                  | 1000/s | 758.77/s (>800.00/s) | 1133.36ms | 1355.36ms (<500ms)    | 100.00% (>95%) | FAILED²

Looking at Grafana dashboard we see that Postgres CPU usage has spiked significantly.

• Latest results Metrics Dashboard - http://50k.testbed.gitlab.net/-/grafana/d/tVdO8YIWz/server-performance?from=1596459947891&to=1596464663230
• Last week Metrics Dashboard - http://50k.testbed.gitlab.net/-/grafana/d/tVdO8YIWz/server-performance?from=1595813448524&to=1595818163214

Comparison with last week results