[INTERNAL] corrected cleanup of document checksums

Commit e892314f8f3c8de98884b9e3248357bcafbc425b messed up
the former cleanup logic. While this does not create any
defects the watchdog server component may waste memory
until the session is stopped.

Change-Id: If375e769a151edb685f1e8aac48a79c38ae9c235
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1398
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>