[FIXED JENKINS-15858] Avoid loading too many build records for displayed jobs

Should more or less fix [JENKINS-15858](https://issues.jenkins-ci.org/browse/JENKINS-15858): load at most ten build records for each job to calculate its status.

Note that the patch forces a core dependency update to 1.509.x LTS, to get the `RunMap.purgeCache` method introduced in 1.507. If you do not want to require new cores for ongoing updates, I can just comment out the relevant parts of the test for now.