[INTERNAL] fixed Cons.WatchdogHandler thread access

The content(s) of the document(s) must be accessed in the UI thread.
This was not the case.

Also fixed a minor memory leak as the document checksum was not
disposed.

Change-Id: I0b5010a1bda854286b69fb42f549db25647ae90c
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1887
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>