[INTERNAL] Don't discard timestamps on checksumActivities

We calculate and compare timestamps for checksumActivities at many
points, but the timestamp was actually set to null as soon as we
transformed them into checksumDataObjects (before sending them).

Also removed the references to readonly-User from the JavaDoc. 

Change-Id: I9b3d00c7bf4b00a851b1c0db891707e26017c8e3
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/858
Tested-by: Jenkins CI
Reviewed-by: Bernd Bieber <brotlee@zedat.fu-berlin.de>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>