* check update timestamps for signatures, user-avatars, ephemeral-settings, last-subject
* check update timestamp for group-avatars
* check update timestamp for group-names
* check update timestamp for memberlist
* check update timestamp for protection-settings
* add a more advanced test
* add another more advanced test
* set last-subject-timestamp more carefully
* bubble up errros from set_*timestamp() and check for from_id==0 before
* simplify Params::set_i64()
* remove comment that is more confusing than helpful
* use update_timestamp() wording consistently
