Timing dependent tests in the IO tests don't work too well when the next
commit increases the JWT parsing performance.
The remaining IO tests are for coverage and basic breakage. Loadtests
are adapted so that performance regressions for JWT caching would be
detected that way.
PR #2358 added a bulk insert to the loadtest. However this broke the regular insert test,
which just returned 400 Bad Request because of a missing PK value since. Adding the new
id column in the payload to the ?columns= argument fixes that.
