GC cleanup the cache too aggressive for submodule builds

Recently trying to stage qtbase 5.12->dev merge, https://codereview.qt-project.org/#/c/243500/ . Due to flaky in COIN, like clients lost connections happened too often. I use status check(background build) more for qtbase builds. But when I cancel the status check, and staged from gerrit again, looks like new build didn't reuse the previous build results.

http://coin/coin/integration/qt/qtbase/tasks/1541439676 and http://coin/coin/integration/qt/qtbase/tasks/web_qt_qtbase_1540355576851

http://coin/coin/integration/qt/qtbase/tasks/1541492038 didn't reuse the previous results