[JENKINS-18368] Diagnostics and workaround for Unix/Windows polling mismatch

[JENKINS-18368](https://issues.jenkins-ci.org/browse/JENKINS-18368) makes the ClearCase plugin useless for mixed Unix/Windows installations in 1.509.2, an effective regression compared to 1.480.3. While I have marked this issue `lts-candidate`, you may consider issuing a temporary workaround. This patch also includes richer diagnostics for polling that helped me track down the issue and verify the fix.