Fix indexing task failure

When running selenium tests on my machine, logs show indexing failures.
Hudson does not seem to have the problem because all its tests pass: mine fail when testing rights blocking (documents security is not reindexed properly).

Problem appears after ~ 10 min of tests, usually at the end of workflows tests. When the problem starts to appears, selenium tests can be stopped: it can be reproduced easily blocking rights on a folder. It disappears when restarting the server.

It seems like a closed or timed out core session id is still referenced in the indexing thread.