update chat/contact data only when there was no newer update (#2642)

* 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

src/lib.rs

@@ -83,6 +83,7 @@ mod simplify;
 mod smtp;
 pub mod stock_str;
 mod token;
+mod update_helper;
 #[macro_use]
 mod dehtml;
 mod color;