The current postStart code holds Jenkins off the "started" state until
Jenkins becomes idle. But it should be enough to wait until Jenkins
start handling HTTP requests to consider it "started".
More reasons why the current approach is bad and we should remove it,
from @coreyoconnor in
https://github.com/NixOS/nixpkgs/issues/14991#issuecomment-216572571:
1. Repeatedly curling for a specific human-readable string to
determine "Active" is fragile. For instance, what happens when jenkins
is localized?
2. The time jenkins takes to initializes is variable. This (at least
used to) depend on the number of jobs and any plugin upgrades requested.
3. Jenkins can be requested to restart from the UI. Which will not
affect the status of the service. This means that the service being
"active" does not imply jenkins is initialized. Downstream services
cannot assume jenkins is initialized if the service is active. Might
as well accept that and remove the initialized test from service
startup.
Fixes#14991.
51e5beca42